Two-third rule
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For safety reasons, railway lines are assessed and a speed limit imposed on trains running on that line. Many factors can effect the speed limit of a particular line including the state of the track, the curvature of the track, the number of stations, the individual features of stations or junctions and the stopping distances of trains running on that track are examples of such factors.
On some passenger lines in southern England, speed limits are higher than they might be elsewhere because the trains which operate on these tracks are usually electric multiple units (EMUs) which are capable of slowing down much quicker than many other trains, particularly freight trains.
On such tracks, the two-third rule applies to freight trains. The two-third rule means that the speed limit for a freight train is automatically two-thirds of the speed for an EMU.
For example, the Dartford Loop Line has a speed limit of 60 mph for EMUs between Lee and Crayford. A freight train on this track is therefore limited to two-thirds of 60 mph, ie 40 mph.
