強者來翻譯,看看你水平（勿用網(wǎng)絡翻譯工具等）
On most modern railways, detection of the train’s position on the line and signal changing are done automatically and colored-light signals have largely taken the place of mechanical ones.
The light signals usually mean:
Green: proceed at line speed.
Yellow: warning-the next signal is at red.
RRailway signaling differs from the traffic lights used on roads in that trains cannot stop quickly; for example, a train traveling at 160 km/h (100 mile/h) would need several kilometers to stop. Therefore on a railway, train need to be given advanced warning of a red stop signal so that they can slow down to a speed where they can stop quickly (a yellow signal). Put another way, yellow signals usually do not indicate an impending change to a red signal at the same location. Rather, a yellow signal gives a train advance warning that the following signal will be red.
To achieve this, the railway is divided into “blocks”, where blocks are stretches of track between two signals. Trains are automatically detected when they enter a block, which is done by what is known as a “track circuit”.
When calculating the size of the blocks and hence the spacing between the signals, the following has to be considered: line speed (the maximum speed the train is allowed to travel); gradient (to compensate for the assistance or otherwise afforded to deceleration); sighting (the ability of the driver to see the signal); reaction time (of the driver).
The track at either end of the block is electrically insulated, and within the block a small electrical current passes though the track. When a train passes a signal and enters a block, the metal wheels and axle of the train short-circuit the current, which causes a relay association with the track circuit to itself become de-energized.
When the relay is de-energized, the signal which the train has just passed automatically turns from green to red, the signal behind that one automatically turns yellow, and the signals behind that one can remain green.
ed: stop.