PNW Railfan's description is correct. While I am not that familiar with the Fairbanks Morse electrical system, what may have sounded like "shifting gears" most likely was the governor and excitation system "unloading" the main generator to reduce main generator current during transition.

Depending upon the circuit, generally the S1 contactor closes to put the motors in series. During transition, the S1 opens and the P1 and P2 contactors close connecting the motors in parallel. In Series, all of the main generator current flows through the S1 contactor and if opened under full load, a HUGE arc would occur drastically shortening the life of the copper contact tips. Reducing generator output allows transition to occur under a controlled condition and the timing and interconnection of the battery field (BF) and shunt field (SF) contactors provides for a smooth re-application of traction current once transition is made.

Dick Samuels
www.oregonpacificrr.com