Passing Danger under ATO

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:

braking rate (usually 0.75 m/s2 in the open and 1.15 m/s2 in tunnels)
a list of block section lengths
a gradient profile
distance to the next station stopping point
distance to the station-skip point (if needed)
up to three sets of coasting instructions (if needed).

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:

a starting location (not necessarily a block section boundary)
a minimum coasting speed
a maximum coasting speed

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.