In response to Sachin's pic showing Shunt removal instructions inside Loco. I hope this will help in understanding working of DC motor Locos such as WAP 1/4 and WAG 5/7. Back EMF is a potential developed across a rotating motor which has reverse polarity to the supply voltage. It has a tendency to reduce the supply voltage which drives the motor. Thanks to Electrical Services Department Engineers for their technical inputs. (anant singh)