The engineer didn't "hit" or "slam on" the brakes, these are sensationalist terms that the modern media loves but they don't mean anything. You're referring to a hard application, and it sounds like it was caused by a PC hit (the noun, not the verb). A PC hit is a fault on the locomotive that results in a power knockout and an involuntary brake application. That would explain the second abrupt stop.
Moderator: Massachusetts Bay Transportation Authority
Avatar:3679A (since wrecked)/3623B (now in service as 3636B).