Moving Block in Communication-Based Train Control: Boon or Boondoggle?
Bill Moore Ede1, Scientist and Alan Polivka, Assistant Vice-President Communications & Train Control
1Transportation Technology Center Inc. (TTCI), Pueblo, USA
Abstract
Moving Block train control has been used to advantage within the transit industry. There is considerable debate within the railroad industry on whether Moving Block train control can be of advantage within a freight rail environment. This paper examines the impact of a Moving Block operation compared to that of Fixed Block, and identifies the enabling technology. The paper concludes that, while the benefits of Moving Block cannot be realized on a continuous basis, there are scenarios in which Moving Block train control is beneficial.
Introduction
The concept of Moving Block train control is one which is applicable to fleeting or following train moves. In this concept, the movement authority limit of a following train is extended nearly constantly to the location of the rear of the leading train. As the name implies, the limit of authority is not just a fixed signal location, but can be at any point along the track. This is a concept which has been used to increase capacity in several transit applications.
This paper examines train headways to understand how a train control system can affect the capacity of a route. First, headways in a conventional signal system are considered. Secondly, what is required to achieve a Moving Block system, including limitations of practical implementations, is addressed. Finally, the headway achievable with a Fixed Block system is compared with that of a Moving Block system, leading to conclusions. Examples are provided from the North American Joint PTC (NAJPTC) project.
Headway achievable with a Conventional Fixed-Block Signal System
For purposes of this paper, “headway” is defined as the shortest time achievable between arrivals at a particular location of the leading end of two successive trains moving in the same direction at the same steady state speed.
In any signal system, minimum signal spacing is based on the normal service braking distance of a worst case train operating at track speed. Frequently, longer signal spacing is used if the traffic level does not warrant close spacing.
Ideally, signals would be spaced so that the train running time between any two of them is the same for a given train. This would mean that signals would be more closely spaced where trains move more slowly, as on upward grades, and farther apart where train s are able to operate faster. However, this is rarely practical from a cost standpoint. Clearly, control points lock in the placement of controlled signals. From a cost standpoint, intermediate signals for each direction need to be located at the same site. As a result, signals are spaced more or less an equal distance apart rather than being based on the operating characteristics of a train.
In centralized traffic control (CTC), when a train dispatcher (train controller) – or automated dispatching logic – needs to authorize a train into a control block, a request is sent to the controlled signal governing entry to the control block. The field logic of the signal system determines how much authority (by means of signal aspects) will be given to the train.
From an operational standpoint, a following train will generally operate far enough behind a preceding train that each successive signal it faces is Clear. If a following train closes up so that it accepts less than a clear signal, (North American) operating rules dictate that the train must slow to 40 mph so that it will end up falling farther behind the leading train until it is operating on a clear signal.
Figure 1 illustrates a fleeting scenario in a four-aspect signal system in which both trains are operating at approximately the same steady speed of more than 40 mph.
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Figure 1: Elements of headway in a 4-aspect signal system.
The elements that make up headway are as follows: 1. The time it takes the lead train to traverse its own length 2. The time it takes the signal system to detect that the lead train has just left the previous block, and to clear the signal the following train is approaching 3. The time to traverse three signal blocks. 4. The reaction time of the locomotive engineer (train driver) who is likely to want several second seconds of a Clear aspect before passing the signal at greater than 40 mph
In a four-aspect signal system, if the lead train is operating at 40 mph or less, the following train can close up and follow on an Advance Approach aspect, reducing train separation by one signal block length.
The key is that in a fixed block system trains are held a constant, worst case distance apart regardless of operating capability, and thus headway is a function of speed.
Headways achievable in a Moving Block System
In a true Moving Block system, there are no fixed points (other than Control Points) to which an authority is issued. Authorities are issued to the rear of the leading train in a fleeting move, rather than to a signal behind the leading train. Such a system requires precise , timely information on the location of the front and rear of each train. Therefore, in a Moving Block system, each train determines its own location and reports it to the safety logic server which is protecting all trains and authorities.
Below is a description of the elements of Moving Block headway followed by an explanation of how the enabling technology, Communication-Based Train Control, works. In the fleeting scenario shown in Figure 2, the elements that make up Moving Block headway are as follows: 1. The time it takes the lead train to traverse its own length plus a safety buffer behind it 2. The time it takes for a location report of the lead train to be delivered to the safety logic server 3. The time it takes for the safety logic server to issue an updated authority 4. The time it takes for the updated authority to be delivered to the following train 5. The time between successive location reports from the train to the safety logic server – this is frequently limited by the communication bandwidth available 6. The time for the following train to traverse the normal service braking distance for its current speed 7. The time for the following train to traverse the buffer distance based on location uncertainty 8. The buffer time that the locomotive engineer allows to avoid riding on the edge and unwanted warnings (generally greater than the enforcement warning time)
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Previous Location Report and Current Authority Limit Current Location Report & New Authority Limit Rear of train when New Authority Delivered to Following Train
Figure 2: Elements of headway in moving block control.
A major variable in the Moving Block headway equation is the normal service braking distance for the current speed. This distance is a function of train speed and of the braking characteristics of the train. Thus, if the following train has better braking characteristics, it can operate more closely. If trains are operating at slower speeds, they can operate more closely. What results is a relatively constant headway over a significant range of speeds.
Before illustrating the differences in headway of fixed and Moving Block, it is worthwhile to examine Communication-Based Train Control, the enabling technology.
Communication-Based Train Control (CBTC)
CBTC goes by many names. Here are a few examples: § Positive Train Control (PTC) § European Train Control (ETCS) § Electronic Train Management System (ETMS) § Incremental Train Control System (ITCS) Some CBTC systems are designed to be overlaid on conventional (e.g., Fixed Block) train control systems, and may be non -vital overlay systems or vital overlay systems Other CBTC systems are designed to be vital standalone systems (which may also have an overlay capability for migration purposes).
A CBTC system must be implemented in a fully fail-safe manner in order to be able to provide Moving Block functionality, which is vital. Figure 3 shows a basic CBTC concept.
PTC Office Server • Tracks Trains Closely More Precise and Timely Visibility • Monitors Field Systems of Operations in Office • Verifies Authority Requests • Transmits Authorities & Restrictions to Trains Onboard Equipment • Determines Location & Authorities & Reports to Server Restrictions • Displays Authorities & Location Restrictions Reports • Warns of & Enforces CommunicationsCommunications Authorities & Restrictions NetworkNetwork Adds to Operational Safety
Authorities & Restrictions Location Reports
6268 UNION PACIFIC
Figure 3: Basic concept of CBTC.
A CBTC locomotive has a location determination system (LDS) on board which determines which track it is occupying and where along the track it is located. The LDS may be a GPS-based system or a transponder-based (balise-based) system. NAJPTC sys tem, for example, uses a GPS-based LDS, including inertial sensors, th at is accurate to 10 feet along the track. Other GPS-based systems are only accurate from 200 to 300 feet along the track, sacrificing precision for simplicity of track database. When a transponder-based system is used, the location computed when a transponder is read is very precise, but as the train moves along the track, the location computed loses precision, due to wheel slip and creep.
The location computed is transmitted by data radio to the safety logic server. In turn, the server will parcel out new authorities to trains based on train dispatcher requests, field conditions, and the updated locations of other trains. If no other train is present in a requested control block, the server can issue authority for the entire control block; otherwise, it will parcel the authority out to the rear of the preceding train, updating the authority limit as the preceding train reports its progress.
From the viewpoint of the train dispatcher, the process of issuing directional authorities is the same in both CTC and CBTC. The difference lies in how the authority is parceled out to the train by the safety logic. Both systems provide for exclusive occupancy for a train within given limits. Both systems provide for authority conveyance, CTC through wayside signal aspects, and CBTC through an onboard display, a sample of which is shown in Figure 4 below.
Figure 4: Example locomotive display conveys authority, restrictions & track profile.
When conducting Moving Block operation on a CBTC system, it is undesirable to display the standard Fixed Block signal aspects because they can unnecessarily restrict train movements. On the other hand, some wayside signals may be required for the operation of trains on which the onboard CBTC system has failed. The NAJPTC project addressed this by having a special signal aspect for equipped trains, and regular signal aspects for unequipped trains.
Issues and Complexities of Moving Block
There are four key issues associated with the implementation of Moving Block which are discussed in this section: § Certain knowledge of the rear end of train location § Communication throughput, latency, and reliability § Conservatism required in enforcement algorithms § Broken rail protection
As the Moving Block concept calls for a following train’s authority limits to be issued to the rear of the preceding train in fleeting situations, the location of the rear of train becomes vital information. This means that the system cannot be expected to rely solely on manually prepared train consist information as the manual processes have not generally been developed to fail-safe standards.
There are, however, a number of potential ways to address this. One way is to measure the train length based on monitoring the time when the train exits a track circuit, particularly a switch occupancy circuit. At that point in time, the onboard system knows where the leading end of the train is. The CBTC system can compute train length knowing the location of the track circuit boundary. This is the approach planned for the NAJPTC system currently under development for initial deployment in Illinois. Other alternatives are a) to put an LDS on the rear of the train with communications to the controlling locomotive’s CBTC system, b) to provide a wayside train length system, or c) in the case of new generation passenger trains, to use information already available on the train.
Communication latency is the second issue. The problem lies in mobile Radio Frequency (RF) communications as one cannot rely on a failure-free first-time message success rate. When a location report message does not get through the first time, it must be re-sent after some delay. This causes a variable message latency which will cause a following train to operate farther behind a preceding train than ideal. This will be highly dependent on the first time success rate – a communication system with a 95% success rate will permit shorter headways than a system with a 90% success rate, all else being equal. Also, the train throughput capacity of a CBTC system is proportional to its data radio throughput capacity. Thus, attention to the RF communication system is extremely important to the design of a smooth running CBTC system.
The third issue is the conservatism required in the enforcement algorithms. One of the objectives of new generation train control systems is to have positive enforcement of authority and speed limits to provide for added safety. This is particularly important when a railway wants to increase the capacity of a route through the use of Moving Block, in which closer spacing of trains is the objective. The problem with trying to calculate the braking distance for a full service application for enforcement purposes is that there are many braking variables that are simply not known nor even measurable. These include such things as brake valve type, brake rigging efficiency, brake shoe composition, piston travel, and actual train weight. Braking algorithms are required to be robust enough that there is only a miniscule probability of allowing a train to get past an authority limit, and therefore they must take into account the probability of bad braking characteristics. This is done by adding margin to the braking algorithm, which must also account for front and rear of train location uncertainties. The warning associated with a conservative braking algorithm may cause locomotive engineers to operate their trains more conservatively than they otherwise would, based on their knowledge of the trains’ braking characteristics.
The final issue is that of broken rail protection. If broken rail protection is required, a full track circuit length in addition to braking distance is required between trains, with sufficient time to detect whether the track circuit clears or not. In such a case, full Moving Block benefit is not achievable.
There are, however, situations where Moving Block may still be advantageous. For example, there have been instances where a train operating at Restricted Speed has run into the rear of a train ahead, because the locomotive engineer did not recognize early enough that the train ahead had come to a stop. In such situations, a Moving Block system would permit a train to operate behind another, and to close up behind it without permitting a collision.
As vital CBTC must be implemented as a closed loop system, operation can be tailored to the situation. For example, if it were determined that passenger trains do not leave behind a (system-detectable) broken rail if operating below a defined speed, or if broken rails are not experienced above a given temperature, the system can be designed to permit Moving Block operation only when the right conditions exist.
Comparison of Headways
Figure 5 compares the ideal headways for the following four situations: § CTC four-aspect system with 1.25-mile signal spacing, 6,000-foot train length, and poor braking characteristics (Fixed Block Signaling) § CBTC system maintaining broken rail protection, with the same pair of trains (CBTC with track circuits) § Pure CBTC Moving Block operation with the same pair of trains, where the maximum location reporting interval is one minute (Moving Block – poor braking train) § Pure CBTC Moving Block operation with the second train having better braking characteristics (i.e., passenger train) with the same reporting interval (Moving Block – good braking train)
Comparison of Train Headways - Signal Spacing 1.25 miles
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12.0 poor braking train – good braking train – 10.0
Headways (min) 8.0 Fixed Block Signaling CBTC with Track Circuits
6.0 Moving Block Moving Block
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Figure 5: Comparison of train headways: Signal spacing 1.25 miles.
The graph shows that the headway characteristics for the freight following train are much the same at higher speeds regardless of the type of system in use. However, as the operating speed is lowered, the headway increases for the fixed block operation, whereas the headway remains more or less constant for the Moving Block operation. At slower speeds even the headway increases as the time for the lead train to traverse its own length becomes predominant. The CBTC operation with track -circuited broken rail protection performs better than fixed signal operation, but not as well as Moving Block.
The fourth vertical bar illustrates the headway for a Moving Block operation where the second train is a passenger train with significantly better braking characteristics. It shows a significantly reduced headway at higher speeds compared to a train with poor braking characteristics operating under Moving Block. At slower speeds, the headway is still less, but it is much closer to that of the heavier train because the time for the first train to traverse its own length prevails.
Comparison of Train Headways - Signal Spacing 2.5 miles
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14.0 poor braking train – good braking train – 12.0
10.0 Fixed Block Signaling CBTC with Track Circuits Moving Block Headways (min)
8.0 Moving Block
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Figure 6: Comparison of train headways: Signal spacing 2.5 miles.
Figure 6 shows the relative headways when the signal spacing is 2.5 miles (a more common spacing) instead of 1.25 miles. It shows that a significant improvement in headways is achievable with CBTC (with or without Moving Block) without having to add additional wayside signals.
As identified earlier in this paper, the headways presented are based on ideal conditions, but the general relationships will hold true for less than ideal conditions as well.
Applicability of Moving Block to Freight Rail Operations
The key advantage of Moving Block is that it allows a following train to operate behind another train with the spacing determined by its own braking characteristics and operating speed. This is in contrast to a Fixed Block system where a following train is held back a fixed distance based on a train with worst-case braking characteristics operating at maximum allowable speed.
While it is true that operating a large number of trains in close fleeting mode is generally not practical, there are, nevertheless, several conditions which can pose a significant operating or safety challenge to railroads.
One of the biggest challenges involves operations around track outages for track maintenance or track renewal. In a double or multiple track scenario, when a track is taken out of service a temporary speed restriction is often placed on the adjacent track, creating a bottleneck situation. As a result, trains traveling in one direction can stack up waiting to get through, while trains travel in the opposite direction past the worksite. Then, the waiting trains are fleeted through while trains stack up at the opposite end. Fleeting trains past the worksite with a shorter headway attainable via Moving Block will reduce the train delay at each end and keep traffic more current.
The situation is much more severe in single track scenarios, where major track maintenance or renewal results in a 6 to 8 hour maintenance window when no trains can pass. Frequently, this is accompanied by a need to store the equipment on an adjacent siding, with a temporary speed restriction over the worksite. If there is no opportunity to detour traffic, there is a build up of trains on each side of the worksite. Recovery in such conditions is often accomplished by fleeting as many trains in one direction as feasible before moving trains in the opposite direction. An ability to space trains more closely during their start-up would allow the operation to recover more quickly.
There have been a number of recorded instances of rear-end collisions between trains approaching and entering yards at Restricted Speed. These have occurred at locations where there are a high number of trains and considerable congestion, and may happen because the train crew in the following train is unable to tell until too late that the train they are following at Restricted Speed has stopped. This sort of situation is not covered by most other systems. However, CBTC with Moving Block capability can display to the following train’s crew the limit of authority which is located at the rear of the leading train. If the train ahead stops, this becomes evident on the crew’s display and the CBTC system is able to enforce a stop if the crew does not react.
Overtakes are a source of considerable delay to lower priority trains particularly when the train to be overtaken is held in a siding until the overtaking train has passed. In a Moving Block control system, delays due to overtakes can be reduced as an overtaking train can close up more tightly behind a train to be overtaken before the latter is put in a siding. In addition, the overtaken train can move out as soon as the route can be lined after the overtaking train has passed, rather than waiting for it to clear a whole signal block.
There are other scenarios that can benefit from Moving Block, where trains operate at slower speeds and capacity is constrained by the signal system. An example would be a passenger/commuter terminal where trains from a number of routes converge.
Frequently, the timing of commuter trains is such that a signal system cannot accommodate threading a freight train between successive commuter trains during rush hour peaks. One of the issues that is raised when new commuter systems are being considered is whether the operation of commuter trains will constrain the freight operation unnecessarily. Because of the frequent stops a commuter train makes, its average speed is relatively low, e.g., 35 mph or less. In most cases, this is an achievable speed for a freight train that would permit it to be threaded between two commuter trains with the close-up capability afforded by Moving Block.
Conclusion
Returning to the question on the title of this paper – Moving Block in Communication-Based Train Control: Boon or Boondoggle? – what conclusion can be reached?
If the expectation is that the advantages of Moving Block operation can be achieved in all locations on the railroad every minute of every day, the answer is clearly “no”.
There are, however, many scenarios where the ability to space trains more closely than a fixed block system allows would be beneficial to train performance and track capacity. The associated enforcement feature benefits the safety of the operation. Railways planning to install a CBTC system should consider ensuring that the design will accommodate Moving Block, whether or not they choose to use it in the first phase of a project. It is always more costly (and difficult) to change a design after it has been installed.