Max,
I've read through the answers to your question and believe they are incorrect about the effect of the compensation winding. Unfortunately I don't have a motor in front of me the confirm this, but if my memory serves me correctly....
The compensation winding in our ET traction motors, which is in series with the armature, weakens the field when the motor is running forward. Without a compensation winding DC motors, running in the forward direction, will slow the heavier the load (current draw) is. If the compensation is set to weaken the field under load the effect is to speed the motor up. GE just had to size the compensation winding to weaken the field just enough that the motor only slowed down a little under load, not speed up.
GE by using the compensation this way reduced tractor speed changes due to hills and the like. I know my E15 doesn't slow all that much going up hill, at least in the lower motor speeds of 1, 2, and 3.
Steve Naugler