Yes, the PTC correctly programmed on board the locomotive would know its location, direction of travel and speed via the speed sensor on one of the axles and also the onboard GPS and the speed restrictions for every inch of that route. Hence, no matter what the engineer did or didn't do, the train would never exceed the speed limit for any part of the route. The failsafe built into PTC would cause a penalty brake application and also cause the engine to go to idle and stop loading. This would cause the train to stop in virtually all instances while the engineer waited to recover from the penalty application, an operation that takes enough time that the train would almost certainly come to a complete stop.
Incidentally, older train control systems would usually have a "suppression" function built in that would allow the engineer to avoid a penalty by applying the automatic train brake if he was pushing his train right to the speed limit and went a bit over. An air whistle in the cab would tell him he was doing that (speeding) and by applying the brakes he would have the ability to slow the train below the speed limit and thereby avoid a costly penalty application. I am not sure if PTC allows an engineer to "suppress" an overspeed penalty but there is a warning beeper or whistle that warns the engineer and he might be able to slow the train quickly enough to avoid a penalty. There's supposed to be no more than 8 seconds from the start of the warning beeper or whistle until the penalty takes place. (that's without the ability to suppress....there was no time limit on the ability to suppress because the train brake was applied and therefore it was assumed that the train must be slowing)
