What determines where locomotives are changed for a train?
Locomotive changes often happen at convenient points where there is an appropriate loco shed where locos can be housed for a while and given some routine maintenance if necessary, etc. Except for long express services (for which keeping down the number of halts is a priority) hauled by WDM-2, WAM-4, WAP-4, WAP-5, and other such locos for which repair and maintenance facilities are provided at many major sheds, generally a loco will not work too far outside the territory of its home shed or zonal railway. If it breaks down too far from its territory repair crew have to be sent out from the home shed to take care of it. Hence, the WCAM-x series locos for instance generally stay close to their home sheds even though they could be dispatched further without any problem of traction change, etc. Another point to consider is whether the loco changeover point is a convenient junction or other station where there are enough trains arriving and departing that a loco can be quickly turned around
