december 2012 | the rail engineer | 51
light rail
writer
Clive Kessell
Victoria Line
Renaissance London Underground’s Victoria W hen Line opened in 1968, it measured 21km long and was the first metro in the world to have Automatic Train Operation (ATO). This was a revolutionary step and the system was designed under the guidance of the legendary London Underground signal engineer, Robert Dell. Using different frequency track circuits for the essential train commands, the system worked well for over 40 years but by the early 2000s the technology looked obsolete and more modern systems of ATO had emerged. The trains, too, were nearing the end of their life and so a complete renewal of the system was planned. Replacing one ATO system with another, whilst keeping the railway in operation, was itself a challenge. A suitable method of doing this was devised and the last old trains were finally retired in mid 2011. So how was it done and what were the challenges along the way? the rail engineer recently visited the Northumberland Park control centre along with a group of IRSE members to find the answers.
Control system As with any replacement system, making the old technology work with the new led to a number of choices. Should the old trains have the new system additionally installed so that when parts of the infrastructure were converted, the trains could operate to the new control commands? This was rejected as too complex and costly. Duplicating the ground based system was the alternative, but could this be done? The eventual solution was to acquire a train control system that could be overlaid on to the existing such that the SIL 4 interlocking and train protection information was conveyed to both the old and new trackside communication equipment in parallel.
The system chosen was supplied by Invensys Rail. At its heart are a number of Westrace microprocessor interlockings that generate the route setting and route holding requirements as well as generating the train movement authorities. Linked to the interlockings is a self-contained radio network, entirely based on radiating cable, which communicates the commands to the trains. The system is termed DTG-R - Distance to Go-Radio - and is fixed block, i.e. there are always fixed distances between succeeding trains, to which are transmitted the distance to go commands. Train detection is achieved through the new FS 2550 track circuits, a standard jointless product from Invensys which has replaced the old coded track circuits once all the old trains were withdrawn. APR (Absolute Position Reference) balises are positioned periodically between the running rails to give accurate information to the system on the position of every new train, this being an integral part of the ATP (Automatic Train Protection) system. These balises are passive devices, obtaining their power by inductive coupling from an undertrain APR reader. A different radio frequency is allocated to each block section, these being in the 170 and 180 MHz band. The new trains are equipped with two side and one roof aerials that receive the command signals in parallel. The ATP system is rated as SIL 4 whereas the ATO equipment is SIL 2. This has become the norm for automatic metro operation. The radio system has been supplied by TE, a UK firm that has established itself in the track-to-train radio market. A base station is located at every signal equipment room with the command signals fed to both ends of the radiating cable via an optical fibre link to keep the system in operation should a cable break occur. This radio system is completely independent of the ‘Connect’
radio network that is in use across London Underground for voice communication to trains and station staff. In the event of Automatic Train Control (ATC) failure, the system can be operated in Restricted Manual Mode at 15km/h under signals. If the train ATP has lost its location information, a minimum number of two APRs are required to be read, plus a confirmation of correct block occupancy, before ATC operation can be re-established. Should the ATO fail or ‘go lost’, the system has the ability to run in Protected Manual Mode allowing the driver to drive at full line speed as if the ATO did not exist. This feature minimises disruption.
In the cab of a Victoria Line train.