英语翻译
Metro Signaling and Modern Railway Signaling
Signaling used on high density metro (or subway) routes is based on the same principles as main line signaling.The line is divided into blocks and each block is protected by a signal but,for metro,the blocks are shorter so that the number of trains using the line can be increased.At first,metro signaling was based on the simple 2-aspect (red/green) system as shown in Diagram 8.Speeds are not high so three-aspect signals were not necessary and yellow signals were merely put in as repeaters where sighting was restricted.
Many metro routes are in tunnels and it has long been the practices of some operators to provide a form of enforcement of signal observation by installing additional equipment.This became known as automatic train protection (ATP).It can be either mechanical or electronic.
Electronic ATP includes tract to train transmission of signal aspects and (sometimes) their associated speed limits.On-board equipment will check the train’s actual speed against the allowed speed and will slow or stop the train if any section is entered at more than the allowed speed.
The Overlap
If a line is equipped with a simple ATP which automatically stops a train if it passes a red signal,it will not prevent a collision with a train in front if this train is standing right beyond the signal.There must be room for the train to break to a stop.This is known as a “safe braking distance” and space is provided beyond each signal to accommodate it.In fact,the signal is placed in rear of the entrance to the block and the distance between it and the block is called the “overlap”.Signal overlaps are calculated to allow for the safe braking distance of the trains using this route.Certainly lengths vary according to the site; gradient,maximum train speed and train brake capacity are all used in the calculation.
Diagram 2 shows the arrangement of signals on a metro where signals are equipped with trainstops (mechanical ATP) and each signal has an overlap whose length is calculated on the safe braking distance for that location.Signals are placed a safe braking distance before the entrances to blocks.