Well... Yes and no...
AC and DC motors being compared are like chalk and cheese in engineering terms.
For example DC motors have two control currents, one to produce the magnetic field and one producing the winding current, the latter is commutated via brushes, exceeding the rating on these would rarely damage the motor unless applied for long periods of time due to the construction.
AC Motors are very different, the only simple way I can think to explain an induction machine is that it follows the phase of the AC supply round the motor, and the rotor is excited with a perminant DC feild, where the stator has a changing feild, and the torque is produced by the difference in angle between the stator feild and the rotor feild, known as slip. As AC motors are also newer technology and usually ran synchronously with the supply frequency (not on trains) the limiting factor would be the construction of the motor.
Today, in the world of variable speed drives (rather than tap changers etc.) the motor doesn't tend to be the weakest link, the power electronics are, and these are very closely rated, and over-running these, even for a short period of time would cause a significant drop in performance, and potential serious fault damage across the components that can damage other system components.
Don't know if that's answered anythin you're asking, but generally, new = AC traction motors, old = DC traction motors. AC controllers much more complex and cannot usually be 'over run'.
