How come Amtrak North East Corridor trains cannot use GE Genesis to travel south of Penn Station?

Whenever I notice a long distance train, such as the Carolinian or the Crescent, how come their journey starts out with an electric train and then changes cabs to a GE Genesis at D.C. How come the whole entire route use the GE Genesis the whole route since they are capable of third rail power.
How come GE Genesis trains are only used on the Empire route whenever they are departing from Penn Station?

The Northeast Corridor is electrified with overhead catenary. By using catenary electric locos like the AEM-7/ALP44, HHP-8 and Acela, Amtrak cuts down on diesel emissions and noise through the densely populated NEC corridor.

Not all Genesis locomotives can use third-rail power. The majority of Amtrak's Genesis fleet are the P42DC, and they cannot use third rail (P42DC is diesel only).

Amtrak only has 20 or so P32AC-DMs, which are the version of the Genesis locos that are third-rail equipped. These are used on trains that run in New York State because diesel power is not allowed in New York tunnels. They can operate anywhere in North America, but Amtrak wants to keep those in New York where they are absolutely required.

The only place on Amtrak's routes that are third-rail-equipped is in New York.. Nowhere else in Amtrak's national system do they use 3rd rail (including the NEC, which is overhead catenary only).

Hope this helps.

Indeed, as thddspc was saying, Amtrak uses electrics when and where possible and typically only switches to diesel power (for commuter/passenger trains) only when entering non-electrified territory. Also, however, Amtrak has a very tight motive power fleet and is always looking for ways to be as efficient as possible (remember, they are constantly on a tight budget).