Dr. Phliskin is the lead researcher on the cargo teleporter project in the main hangar of the Olympus. After the quest 

"Beam Me Up" has been completed, Dr. Pliskin can be found in the room with the cargo teleporter in it. Talking to him gives you the options to teleport to several different locations:

Rivet City

Megaton (If you chose not to destroy Megaton)


Tenpenny Tower

After you have invested the 10,000 caps Into Project Poseidonis, you can take the teleporter to the city of Poseidonis, the capital of the TSC.