In addition to Amtrak, WAS also has a Metro Station, the main intercity motor coach station for DC, and about 3 or 4 car rental companies. I recently returned from a business trip that required driving rather than flying or taking the train, and my travel office booked my rental car out of Union Station instead of the local Enterprise outlet 6 blocks away. I was planning to get out of town during mid-day in order to avoid the notorious traffic in our Nation's capital.
At the appointed time, I left the office, rolled my bag to Metro Center, and caught the Red Line to Union Station. I had no problem working my way up various escalators to the Hertz desk, where the nice woman working there instructed me on how to find my car. This involved going more escalators to the parking garage, the lower level of which is the new bus station, then walking on an outdoor passageway towards H street (in the direction of the DC streetcar) and taking a small elevator at the end to the rental car level. This was reasonably well marked, except that the sign outside the elevator said take it to the "Mezzanine" level, whereas inside the elevator, the only floors marked were "1" and "2." This did puzzle a party of British tourists with whom I shared the lift, but it didn't take too long to figure out we just wanted to go up a level. Once discharged, the rental boots were right in front of us, and I was soon behind the wheel of my car, and on my way out of the city.
As far as the bus station, the whole thing is basically outdoors. Of course, most is protected from the weather by the upper levels of the parking garage, but there are a few places where you can get rained on (like the passageway to the rentals cars, and the loading platforms for the Bolt Bus. Also, there is no indoor waiting area, (though Greyhound has an indoor ticket office), which could be a problem during a typical DC summer, or during the chilly DC winters, which for the past several years have seemed to last into May. I hope somebody has plans to fix these little flaws.
As for car access, There is a traffic light on to H Street, and it was easy to leave. However, when returning the car, some bright person decided to have the tail end of the taxi line sitting on H street at the parking lot entrance. There was a radio-equipped dispatcher there letting them through in dribs and drabs, presumably to enter, and then exit the other side to get to the front of the station for picking up fares. However, there presence made it unclear how civilian cars wishing to enter the parking lot were to proceed. I was a little leery of turning right into the garage from the left lane of H Street, but that's essentially what I had to do. Also, the sign should have been more clear that this was the entrance to the Union Station Garage, and a sign saying "rental car return" on H street would also have been helpful. (The signage in the garage was perfectly adequate.)
Anyway, while it's nice you can rent a car at Amtrak, I don't think I'll deal with DC traffic. The next time I do this, I'll just rent near my home, or at one of the airports.