The Central Line of London Underground has been resignalled relatively recently, and uses a new form of Automatic Train Protection and Automatic Train Control.
ATP (Automatic Train Protection) forms part of the signalling system, and it has greatly influenced the way in which the line is signalled and operated. The system assumes that trains normally operate under the control of the system, whether being driven automatically or manually (a setting known as "coded manual" for reasons that will become apparent).
The line is divided into block sections. These sections are relatively short - as little as 50m at stations, 100m to 200m in the central area, and several hundred metres in outlying areas - and in particular are much more closely spaced than conventional signals would have been. The sections are defined in two ways. On plain track a coded signal is injected into the rails at the end of each section by a signal generator and is picked up by a receiver at the start of the section. A specially tuned arrangement of bonds between the two rails stops the signal being picked up in adjacent section, meaning that gaps are not required in the rails.
The diagram shows two adjacent block sections and the ends of two more. When a train is in the section it short-circuits the rails, which means that the receiver will not pick up any signal, while the train itself detects the signal with a detector mounted ahead of the first pair of wheels; note that this means the generator must be ahead of the train, and so a different pair of generators and receivers must be used when running the other way along bidirectional track. The train equipment is capable of handling the gaps between the block sections.
At points and crossings it is not possible to use this system. Instead conventional track circuits (with insulated joints in the rails) are used to detect the train. Cables are laid just inside the rails and these carry the coded signals; at a divergence separate cables are used for each branch, switched on and off according to the setting of the points.
Each signal generator produces a carrier frequency. The train equipment requires the frequencies to appear in a certain order: either f1, f3, f6, f1, f3, f6, ... or f2, f7, f5, f2, f7, f5 .... Adjacent tracks will use different patterns, and if the train picks up an unexpected frequency - or if no signal is received for more than the metre or so length of the gap between sections - it will stop immediately; this situation indicates either that a signal is being picked from another track or the train is running on the wrong line. Frequency f4 is used to break the pattern when necessary, usually at points and crossings - it is always acceptable and can be followed by any of the other 6 frequencies. Frequency f9 is also used for this.
The base frequency of each block section is frequency-shift-keyed with one of 14 codes. These codes tell the on-train equipment two things: the speed limit in this section and the target speed on entering the next section (usually slightly less than the limit in that section). The code transmitted in the block is based on the state of the line ahead. Thus as a train approaches the point it needs to stop, the codes indicate a gradual decrease in speed:
Note how the codes change as the train ahead moves away. The codes are always arranged to provide a clear overlap beyond the end of a section which is appropriate for the current speed limit of that section.
The 14 codes are as follows (all speeds are in km/h):
|Limit||Target speed||Alternative target speeds|
The alternative target speeds are used to provide additional combinations. Spot loops at the start of block sections transmit one of six frequencies. These tell the on-train equipment to apply the normal target speed for a code (frequency A or no spot loop) or one of the five alternatives (B to F). The loops switch frequency according to requirements.
In addition, in the tunnels east of Liverpool Street a curious arrangement is used to achieve a de facto limit of 80 km/h. There is a group of three short track circuits, with the first two set for 109/102 but with coasting applied and the third to 109/68. When a train is approaching the end of the third one and its speed has been proven to be below 80 km/h, it resets to 109/102. Coasting then turns back on at the next set of three track circuits.
Trains have a master switch in the driving cab that can be set to one of three positions:
In Restricted Manual mode the train cannot exceed 18 km/h and the motors cut out at 16 km/h. The ATP and ATO systems are disabled and the driver drives entirely by sight and according to the signals. This mode is used in depots or when the ATP system has failed in some way, and is subject to strict rules. A flashing yellow light on the front of the train indicates this mode.
In Coded Manual mode the train is being driven by the driver, but the ATP is detecting the codes in the track and restricting his or her actions. The train speedometer (in 1992 Tube Stock, at least) is of the horizontal strip type and shows two speeds - the current speed above in green and the target speed below in orange. Whenever the target speed changes (either at a section boundary or because the code being transmitted has changed) the driver hears a chime - upwards or downwards as appropriate. If the driver exceeds the speed limit for a section, the train will brake automatically until it is both below the limit and the driver presses a flashing "Alarm" button.
In Automatic mode the train is driven by the ATO system. The driver simply opens and closes the doors and presses the twin "start" button when the train is ready to leave the station. All other operation, including stopping the train at the next station, is done by the automatic systems (though of course the driver has to remain alert in case of obstructions on the line, passengers trapped in the doors, and so forth).
In all three modes the tripcocks (fitted for use on other lines) are disabled.
A few signals remain installed on the Central Line, mostly at the end of station platforms; the location of the signal is always the start of a block section. The aspects shown are different from normal LU signals.
The meaning of the aspects is:
Where there is no signal, the start of a block section is indicated by a block section marker:
At junctions the route to be taken is shown by a white arrow above the signal:
(unlike normal LU practice, an indication is shown for every route) or by a theatre indicator showing the track or platform number. At the entrance to and exit from depots, the change between coded and uncoded track is show by appropriate marker boards:
The normal LU shunting signals and rail gap indicators are used, though the former are now all of the fibre-optic type.
As already described, the ATO (Automatic Train Operation) system drives the train when in Automatic mode. The system takes information from two places: the on-train ATP systems and the Platform ATO Communications (PACs). A PAC is a small transmitter that sends information to a receiver on the ATO. They are located at each station and at the exit from depots and sidings. When the train is standing at the PAC it is sent a description of the line up to the next two stations:
The PAC also proves to the train that it is in the correct place to stop, and provides Correct Side Door Enable information.
In normal operation with a clear line the ATO will calculate the point at which it will need to brake for the next stop; it will then drive the train as fast as possible to this point while remaining within the speed limit given by the code being picked up. If the code gives a target speed below the speed limit, the system will brake the train as late as possible so as to reach the target speed 5 metres before the start of the block section (13 metres if the target speed is "stop"). If the code changes the train will alter its braking or resume acceleration as appropriate.
Under some circumstances the train does not stop at a station (usually because the station is closed). In this case the PAC data includes a station-skip point. The train will be braked so as to attain 20 km/h at this point, and will then continue at this speed until the rear of the train passes the station stopping point. Because only two interstation runs are included in the PAC data, only one station can be skipped.
The ATO normally drives the train as fast as it can. If trains are bunching up or otherwise need to be slowed down, the control room turns on the coasting facility at the PAC. This then transmits one or more sets of coasting instructions to the train. These instructions consist of:
Coasting starts at the specified location and continues until the next set of coasting instructions applies or until the train must brake for a station stop. The train then coasts until its speed drops to the minimum, after which it accelerates again until reaching the maximum and can coast again. By suitable adjustment of the instructions the train can be retarded to a lesser or greater extent.
ATO was brought into use on the following dates:
|Epping to Leytonstone||2001-05-01|
|Woodford to Gants Hill via Hainault||2001-05-01|
|Woodford bay platform||2001-11-16|
|Gants Hill to Wanstead||1999-12-16|
|Wanstead to Mile End||2000-07-17|
|Mile End to Liverpool Street||2000-02-12|
|Liverpool Street to Bond Street||2000-03-22|
|Bond Street to Shepherd's Bush||2000-04-05|
|Shepherd's Bush to West Ruislip||2001-03-07|
|North Acton to West Acton||2001-03-07|
|West Acton to Ealing Broadway||2001-03-07 eastbound|
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