BBWells & Louis:
On the Northeast Corridor, top speed is 150 mph, (in New England), 135 mph, between Trenton, NJ and Edison, NJ and 125mph elsewhere. There are of course restrictions for curves and other considerations. The Harrisburg - Philadelphia "Keystone Line" tops at 110 mph, as does a portion of the Empire Service Line in New York State. Generally, Acela Service does the top speed, while Northeast Regional runs at 125. New Jersey Transit Commuter trains regularly run at 100 MPH, where permitted.
The rules and practices are pretty much the same on all railroads, although the semantics used may differ. Maximum speed, is generally determined by class of service (Freight - Passenger), physical characteristics, class of track and signal system in effect.
Before I retired, I ran diesel powered Keystone trains, (P42 and Amfleet) at 110 MPH. I also ran Corridor trains at 125mph,
Joe