This throttling back of locomotives may be related to
this thread.
Just ran another test train going up a hill at full throttle. Saved the activity. Then resumed the activity after waiting about 15 seconds for the frame rate to get settled before pressing the ESC key to actually resume the activity.
The locomotives actually throttled back (not just the sound of locomotives throttling back) with the train speed dropping fast while throttling back! But a few seconds later the locomotives throttled back up to maximum and the train managed to pick up speed before any stall occurring.
For very marginal situations where activities are resumed for trains running at full locomotive power up hills and at very low speeds (close to stall), I suspect those trains are more likely to stall because the locomotive power can't be brought up fast enough to prevent stalls.
Further testing is in order I believe to check that out for certain.