英語翻譯
Though there is no train is occupying the overlap track circuit.This will give rise to two red signals showing behind a train while the train is in the overlap.
Automatic Train Protection
To adapt metro signaling to modern,electronic ATP,the overlaps are included into the block system.This is done by counting the block behind an occupied block as the overlap.So in a full fixed block ATP system,there will be two red signals and an unoccupied,or overlap block between trains to provide the full safe braking distance,as shown in Diagram 5.By the way,many ATP equipped system do not have visible lineside signals because the signal indication are transmitted directly to the driver’s cab console (cab signaling).
On a line equipped with ATP as shown above,each block carries an electronic speed code on top of its track circuit.If the train tries to enter a zero speed block or an occupied block,or if it enters a section at a speed higher than that authorized by the code,the on-board electronics will cause an emergency brake application.This is the system used by London Underground for the Victoria Line from 1968-the first completely automatic,passenger carrying railway.It was a simple system with only three speed codes-normal,caution and stop.Many systems built since are based on it but improvements have been added.