Automatic train protection is speed and distance supervision, usually intervening (usually deploying emergency brake, as a last measure) when the driver of a train omits to react on optical signals given from the wayside system.

ATP can refer to protection systems in general, as well as particular implementations.

ATP is given permitted speed and location information from the track via encoded balise(s), encoded track circuit or more recently via radio.

Ericsson Ebicab 800 ATP

The Ericsson ATP consists of

• balises (beacons) mounted between the rails at signals and other places of interest
• an antenna mounted on the front vehicle of the train.
• a controller unit mounted in the cab of the front vehicle where the driver can set it.
• connections from the controller unit to the train brakes.

As the train passes the balises, power is transmitted to the beacon, which responds with information on:

• signal aspect
• curve and gradients for a sufficient distance in advance.
• speed restrictions for various types of trains for a distance in advance.

The controller in the cab calculates the maximum current speed and a target speed for a distance in advance.

• If the driver is alert and controls the train speed appropriately, then the Ericson ATP system sits quietly in the background.
• If the driver fails to regulate the train speed appropriately, then the ATP system applies the brakes partially or fully so as to achieve a safe speed.

• At the commencement of a journey the driver must set the cab controller with:
  • the type of train which indicates its braking characteristics, etc.
  • the length of the train (train must go train length past speed limit before limit lifted).

The Ericson ATP is a continously intermittent system, in that the cab controller is active all the time, while the signal and speed limit information is update only at intervals. This is a small disadvantage in heavily trafficked areas, because a driver may see a signal change up to green long before he reaches the balises for that signal which only then allow an increase in speed. This limitation can be partially overcome by additional intermediate balises, or by installing a radio link that transmits a "Signal changed up to green" message.

• When a train comes to a standstill at a red signal, it usually is not located over any balises. Thus when the signal changes up to green, there is no way that the system can know this - hence reliance is put onto the driver to restart the train appropriately, and the driver is given a 40km/h speed margin to power through that signal. This is another case where the radio update is useful.

Taken together, the Ericson ATP and the Radio Update provide continous coverage continuously.

• Note that balises come in pairs so that it is possible to distinguish direction AB from direction BA.

See Also

• Automatic Warning System
• Automatische treinbeïnvloeding (ATB) – a Dutch system which would have prevented Harmelen train disaster.