Rail Engineer • July 2016
For example, a 420 code would indicate maximum line speed whilst a brake spot command would be turned on if the train was required to stop at a signal or headway post (a ‘block marker’ in today’s parlance). However, if the brake spot command was ignored or missing, and the train reached the next track circuit at full speed, this code would already be set to 180 thereby providing the ATP function by immediately enforcing the braking to ensure the train was brought to a stand at the end of the movement authority. Other command spots were located on the approach to and in platforms to ensure accurate stopping at stations.
The new order A fleet of 47 new Bombardier eight-car ‘09’ Tube Stock trains are now in service. David Waboso said that buying 47 trains was a good decision, essential to achieving the throughput of a train every 100 seconds. The Westinghouse Rail Systems (subsequently Invensys, now Siemens Rail Automation) Distance To Go - Radio signalling system (DTG-R), and ‘Trackguard Westrace’ interlockings were chosen for the VLU. Train detection utilises FS2550 track circuits and signals, mainly consisting of platform starters, are ‘Westled’. Point machines installed as part of the Walthamstow crossover renewal project are the new Siemens ‘Switchguard Surelock’ mechanisms. This machine was developed for LUL as a quick-to-maintain modular package. Incidentally, in a separate exercise, Style M63 point machines, a Westinghouse 1960s-design still in widespread use, will be replaced by Surelock on the Central line, SubSurface Railway, Jubilee, Northern, and Piccadilly lines by 30 April 2018. Train positions are reported to the Westrace interlockings and control centre screens by occupation of the track circuits. Routes are set in the usual way according to the timetable. Westrace then passes Block Proceed (BP), Block Not Occupied (BNO) and Overlap Clear (OLC) to the fixed block processor (FBP), the purpose of which is to package the information in order to transmit it safely to the train via the radio system.
The radio transmission system consists of fixed communications units, trackside antenna subsystem (TASS), mobile antenna subsystem and mobile communications units (MCU). Generally TASS uses radiating cable in the tunnels although at the depot, the only part of the line which is above ground, a free space antenna is used to cover the extensive layout. ATP receives BP, BNO and OLC from the MCUs and determines, from geographic data, the limit of movement authority (LMA). Algorithms continuously monitor the progress of the train and ATP will apply the emergency brakes in the event of a digression. ATP determines absolute train location from Absolute Position Reference (APR) transponders in the track, each of which has a unique identifier code read by APR readers as they pass over them. Between APRs, speed and distance travelled are determined diversely from speed probes and doppler radar units. The LMA is passed to the ATO, which calculates the braking curve and drives the train as closely to the ATP limits as possible without provoking an intervention.
ATO on ATO upgrade Changing the entire train fleet and the entire signalling system presented an interesting challenge. Essentially, the line was to remain operational during the day throughout the project. There is insufficient space at the Northumberland Park depot to store two complete train fleets, thereby ruling out changing over the stock in one hit and thus requiring incremental introduction of the new trains. Fitting the 1960s train control to the new fleet was ruled out given the difficulty of replicating obsolete electronic kit. Installing DTG-R on the old trains was deemed an unnecessary expenditure. The option chosen was to overlay the new DTG-R trackside equipment on the original signalling system and transmit simultaneously the new radio messages to the new trains, and legacy track codes to the existing stock, during the upgrade works and changeover of the rolling stock, a process which took nearly two years. Train detection, point control and detection, and interlocking functions continued to be delivered by the legacy signalling system with the track circuit codes and command
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Driver pressing the twin 'start' buttons.