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.
