Except for the main passenger lines in the Northeast, which use electric locomotives and Self-propelled electric rail cars, the rest are Diesel-Electric, they use a Large diesel engine (Amtraks are either 4000 HP (GE Dash-8-40P) or 4200 HP (GE P42) to run an Alternator, which powers electric motors on each axle. To furter muddy the waters from New England and Albany, and MTA they ues 3200HP Diesel-Electric, Electric locomotives that can switch off the Diesel in the tunnels in NYC and run off the third rail.
Freight locomotives are sized from less than 1000 hp all the way up to 6000, but the, most common is 4400 such as the GE AC4400 or EMD SD-70, locomotive larger than that have proven to be unreliable and more power than 6 wheels can effectivly use without slipping on the rail.