|The following is based on information from Van Buren design documents and is not canon.|
In TTW you have to get to The National Archives. At the back of it there's a station called Penn Ave/The Mall Metro. Go in and go down the escalator and through the only opening in the station and the only door to to Judiciary Square Metro. Once you reach the station kill all the mutants and radroaches. Interacting with the door will start a quest called Union Station. Go down to where the reactor is marked and open the terminal. Select start reactor and a quest will appear and say reprime reactor and (optional) obtain reactor primer. Go down stairs and open the door and grab the primer which is a green substance on the locker. Use it to prime the reactor and start it using the terminal on top.Tthen go back to the main station and turn on all breakers, 1 is at the restaurant. Once all breakers are on go and view the train schedule. Just look at the train schedule and that should be it the door to the train should be open.