The train now standing has automatic doors

Why is it that the person nearest the door on the train always presses the button before the light comes on, then jerks his finger backwards and forwards like it’s going to make a difference? And another thing: when you’re in a hurry, why do the doors stay open for ages while other people dither about, but when you are smartly stepping up to the train they slam in your face or trap you in their evil jaws while you squirm for some vestage of dignity. Despite being a huge source of irratation, however, sliding doors do speed up departures at stations and turn-round times at destinations.

There are two types of automatic door: not, suprisingly, the good and the evil, but rather sliding and sliding plug, operated by either electrical or pneumatic methods. About five years ago, according to SMC Transit International, the most popular method was pneumatic. Now the two are equally specified.

Sliding doors open by sliding into recesses in the exterior of the car, or into spaces within the vehicle body. Sliding plug doors are widely used in mainline, Metro, and LRV (light railway vehicles). A swinging hinge controls the unplugging motion of the door mechanism, moving the doorleaf out and keeping it parallel to the bodyside during further motion. The lower edge of the doorleaf is guided by a swing arm with three rollers. As they close perpendicular to the vehicle body the weather seals are com-pressed to give an effective seal.

In the closed position the doors fit exactly flush with the vehicle body exterior. They are not affected by pressure pulses on high speed trains operating between 140km/h and 300km/h and offer reduced wind noise and drag, and neat appearance.

Power for opening both types of door can be either pneumatic or electric. Pneumatic systems are very reliable. Doors can be operated by independent actuators, with or without synchronising drive belts, or by twin overhead actuators.

An electrical system is simpler, parts are easier to replace and more detailed diagnostics are available. Of the two types of electrical systems, leadscrew and belt drive, very little maintenance is needed but the leadscrew does require regular lubrication.

Some users feel that the rubber belt is the weak link of the belt drive. However, SMC has had no problems with the belt and has never replaced any, even during the 1.5 million door operating cycles regularly carried out on the test rig.

Both the pneumatic and electrical systems feature safety systems, such as obstacle detection, anti-slam, pull back, reduced closing force, interior and external emergency operation, and traction interlock.