Train protection and warning system (TPWS)

TPWS is a development of, but not a replacement for AWS. Its primary function is to minimise the risk of trains passing signals at danger but is also currently used to reduce collisions with buffer stops, and overspeed derailment risks on the national rail network. Equipment is fitted to over 12,000 signals, speed restrictions, and buffer stops and to all trains.

TPWS automatically applies a train’s brakes if it approaches a fitted signal at danger too fast or fails to stop at a signal set at danger. It also automatically applies the brakes if a train is travelling too fast on the approach to certain speed restrictions and buffer stops.

The system is not designed to prevent signals passed at danger (SPADs), but to reduce risk by bringing trains running at speeds up to 70 mph to a stop within the signal overlap. The signal overlap is the distance, typically 180 metres, beyond a signal before a point of conflict is reached.

It is designed to provide a safety margin in the event that a driver misjudges a brake application. An enhancement of TPWS, TPWS+ extends the same level of protection to trains travelling at up to 100 mph. TPWS+ uses the same equipment as TPWS, but in a different configuration.

The capability of TPWS and TPWS+ depends on the speed and braking performance of the approaching train. Where braking performance is reduced  or there are poor rail conditions, overrun distance increases.

TPWS is not a fully automatic train protection system (ATP). Particularly on high-speed lines, ATP remains a long-term goal for the industry.

Last updated: 15 February 2008

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