Moving Block in Communication-Based Train Control: Boon Or Boondoggle?
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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. 4 3 2 1 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) 2 8 6 5 1 3 7 4 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 Network Network 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.