ONE of the issues facing the US rail freight industry is the current and future capacity of high-volume corridors. Publications and presentations are available that provide historical data of capacity as well as maps of parts of the country which show where volume is in highest demand. These publications also project rail freight growth and show future corridor volumes.
Currently, Positive Train Control (PTC) is being implemented in the United States under the Rail Safety Improvement Act of 2008. PTC infrastructure integrates with conventional signalling and consists of four segments that overlay the conventional signalling architecture:
- a locomotive onboard segment capable of automatic enforcement of movement authorities and speed restrictions
- a back-office segment that interfaces railway systems and provides data
- a wayside segment that provides status of wayside devices, and
- a wireless communications segment that connects the other three.
The advantage of moving block compared with fixed block operation is illustrated in Figure 1. Moving block operation will theoretically allow the following train to be at its braking distance, with some additional warning distance and margin, from the leading train. Track circuits can be developed for moving block operation. Conventional track circuits typically provide binary information for a fixed block. The track circuit can either be “clear” - no occupancy and no broken rail, or “not clear” - occupancy and/or broken rail. Figure 2 illustrates a conventional track circuit for a bi-directional track.
Each side of the track circuit will transmit (Tx) and receive (Rx) a signal through the track. A conventional track circuit will identify the block as being clear at Location A if Rx2 receives from Tx3. Similarly, the track circuit will identify the block as being clear at Location B if Rx3 receives from Tx2. This usually includes track signal coordination so only one side of the block is transmitting at a time.
The status information for this block is transmitted through the rails to adjacent track circuits with signals Tx1 and Tx4. An adjacent block uses this status information along with what it detects within its own block to determine which signal aspect to display.
With today’s PTC and proposed future train control systems (especially those that are communications-based), the track circuit information may be transmitted to an office server, or directly to the locomotive’s onboard computer Figure 3.
In the next-generation track circuit, broken rails are similarly detected by monitoring the transmission current. Figure 4 provides an example of the current loop with the transmitted Tx2 signal. If the signal Tx2 is being transmitted and the current in the loop is substantial (I = 0), then the track circuit is clear of broken rails within that current loop. If the signal Tx2 is being transmitted and the current loop is near zero (I = 0), then there is a broken rail. It is near zero since there is likely to be a negligible amount of current flowing through the ballast. Consequently, this method allows for the detection of broken rails even with a shunting axle on the block, but does not distinguish whether the track circuit is occupied or unoccupied if no further information is available.
Additional information is obtained by monitoring voltage by the next-generation track circuit. If the voltage detected at A and/or B drops to approximately zero, but the current in the block is substantial, then an occupancy can be determined to be present in the block. This produces additional binary information from each end of the block. Table 1 presents the various combinations of information provided by voltage and current for each end of the block. The next-generation track circuit can also be used to detect an open or shunt caused by a device such as a turnout or track obstruction detector interfaced with the track circuit. In the table, 0 means near zero or a defined threshold.
Future methods
The context for the next-generation track circuit is that train separation is controlled by future methods of train control like moving block. The train control system will separate the following train from the rear of the leading train based on the following train’s braking distance plus warning distance. Modern train control systems use movement authorities and/or stop targets to ensure train separation. In order to apply the proposed next-generation track circuit, a number of rules should be considered.
A movement authority rule could be designed into the system to account for the situation when a following train enters an occupied detection block; thereby masking the broken rail protection between trains (Figure 5). Masking is possible if the braking distance for the following train is less than the detection block length. To restore broken rail detection, a stop target is held corresponding to the last reported end-of-train location of the leading train at the time when the following train enters the occupied block, when no rail break has been detected in the occupied block until that time. Entering an occupied block can be determined by a database with detection block boundary locations and the known head of train location.
The stop target would be replaced with a new stop target behind the latest reported leading train location once the leading train clears the block and the track circuit determines there is no rail break in the block ahead of the following train. Clearing an occupied block can be determined by a database with detection block boundary locations and the known rear of train location.
The proposed movement authority rule would be designed into the system to protect the following train in the case of broken rails between trains (Figure 6). The 
office/server would need to process the following: the last reported end-of-train location of the leading train at the time when the transmission current behind the leading train changes to I = 0 amps. Once this state change occurs, the corresponding location becomes the limit of the movement authority for the following train.
Additional broken rails could occur beneath the leading train. The first detected broken rail will remain as the limit of the movement authority since that is the only possible instance when the transmission current Tx changed to I = 0 amps.
PTC territory and high-capacity lines are areas that could benefit from future train control and next-generation track circuits. PTC territory provides available infrastructure to leverage, including a wireless network, back office servers, and an onboard database, while high-capacity lines can take advantage of future train control to increase volume and reduce delay. When combined with train location data, the concept will allow the location of a rail break to be identified more precisely.
