Why does a goods train sometimes move backwards briefly before starting to move ahead from a stop?
There are a few different reasons that this happens. One reason (and the official one stated in working timetables) has to do with ensuring the couplers (CBC’s) along the rake are all engaged and locked before starting off. The backward push forces the couplers to engage if they are loose, not fully engaged, or if the coupler pins had been inadvertently (or maliciously) lifted while the train was stopped. Another reason is to compress the couplers along the length of the rake, so that when the loco starts moving forward, it has an easier time setting the wagons at the front in motion first before the rear wagons as the slack in the couplers plays out along the length of the rake — it doesn’t have to set the entire train in motion all at once. This is more important with poor track conditions where the loco cannot develope its full tractive effort before its wheels slips, or with older style bearings on the wagons which have much higher starting friction than the rolling friction encou
