Millions of passengers board subways every day and usually take safety for granted — and with good reason. They are among the safest modes of transportation in the wold. One reason is the multitude of checks and balances that have been designed into transit systems.

London's Underground is no exception. Throughout the system are signals that indicate whether a track ahead is unobstructed, so the train can proceed. If a track is clear, a green signal shows; if it is not, a red signal is indicated. But these signals do not guarantee that a train will stop when it is supposed to. To do this, a Train Stop Motor is incorporated into each signal.

A Train Stop Motor has an external lever that normally is positioned downward. This lever is actuated downward by a pneumatic cylinder. If a red signal is indicated — or if electrical power or pneumatic pressure is lost — a loss of pressure in the cylinder allows springs to pivot the lever upward. When the lever is up, it will physically disengage electrical contacts of any train that passes by — preventing it entering a dangerous situation.

Manufactured by Midland Pneumatic, Wolverhampton, United Kingdom, the single-acting pneumatic cylinder is designed to overcome spring force when air pressure is approximately 5.5 bar (80 psi).

Whenever a red signal occurs, compressed air bleeds from the supply line feeding the cylinder. An extended rod gland prevents side loading that could otherwise shorten the life of piston seals — a feature that contributes to these failsafe actuators operating reliably for 10 years or one million cycles.

This information was provided by John Arrowsmith, of Midland ACS, West Midlands, United Kingdom.