The Train Protection & Warning System (TPWS) is a train protection system used throughout the British passenger main-line railway network, and in Victoria, Australia.

The British Rail Safety and Standards Board's definition is:

The purpose of TPWS is to stop a train by automatically initiating a brake demand, where TPWS track equipment is fitted, if the train has:

• passed a signal at danger without authority
• approached a signal at danger too fast
• approached a reduction in permissible speed too fast
• approached buffer stops too fast.

TPWS is not designed to prevent SPADs but to mitigate the consequences of a SPAD, by preventing a train that has had a SPAD from reaching a conflict point after the signal.

A standard installation consists of an on-track transmitter adjacent to a signal, activated when the signal is at danger. A train that passes the signal will have its emergency brake activated. If the train is travelling at speed, this may be too late to stop it before the point of collision, therefore a second transmitter may be placed on the approach to the signal that applies the brakes on trains going too quickly to stop at the signal, positioned to stop trains approaching at up to 75 mph (120 km/h).

At around 400 high-risk locations, TPWS+ is installed with a third transmitter further in rear of the signal increasing the effectiveness to 100 mph (160 km/h). When installed in conjunction with signal controls such as 'double blocking' (i.e. two red signal aspects in succession), TPWS can be fully effective at any realistic speed.

TPWS is not the same as train stops which accomplish a similar task using electro-mechanical technology. Buffer stop protection using train stops is known as ‘Moorgate protection' or 'Moorgate control’.