Is Traction Drive a Constant Torque Load?

You are right on both counts. It is due to condition of almost like a short circuit present, at the starting of the traction motor, due to requirement of very high starting torque.

A locomotive drive system is rather complex and has to be able to function at two extremes of operation.

One is being able to start at 100% amperage capacity at zero RPM's which produces the peak torque but before the train has began to move. To do that the traction motors are running at low input frequency and low input voltages that are only high enough to get them to draw the 100% amp load.

The second extreme is when moving at high speeds. At that point the traction motors will to be running at 100% input voltage which will allow them to work at frequencies above their nominal ratings to get their RPM's up but when doing so the input amperage values are usually much lower that those experienced at start up.

In both instances the total mechanical effort output can be seen as a simple volts x amps = traction power. One way produces maximum output power with a low speed and high torque the then produces maximum speed with minimal torque but in each case the total power being put out is about the same.