PS1 has no speed control. For a given applied voltage and load, the motor will turn at a particular rotation rate (RPM) and produce a fixed torque and locomotive speed. If the load increases (going uphill or around a curve) some of the torque is used to oppose the effect of gravity (climbing the hill) or the increased resistance (on a curve) and there is less torque available to maintain the speed, so the engine slows down. The opposite is true when going downhill and the locomotive speeds up. I run numerous MTH PS1 locomotives. When they are well broken-in, the internal friction is decreased and they run well at slow speeds and also maintain more nearly constant speed on curves and grades.
If your locomotive slows down excessively on grades, make sure it is well lubricated and has good traction tires.