one of railroading great efficiencies, slack (that 2 mile freight train already starts one car at a time)
Is that right? Maybe before roller bearings, when overcoming starting friction was a huge problem.[1] But all US railroad cars in interchange have had roller bearings since about 1992.
Normal starting procedure is to move the engine very slowly until the end of train device reports motion, indicating all the slack has been taken out. Then apply power.