You might want to check out Bolt Bus. Between DC, Philly, NY, Boston it is now my preferred method of travel. Power outlets, wireless for the laptop, trips every hour, good city center locations, online booking, and super-low prices make it essentially unbeatable.
Edit: I'm not arguing bus over train; just trying to help folks out by presenting an interesting alternative
Try traveling those routes on a holiday weekend when traffic on the highways is barely moving. Even without heavy road traffic, though, the train is faster. Plus you can get up and walk around on the train, there's more legroom, and food service.
Edit: I'm not arguing bus over train; just trying to help folks out by presenting an interesting alternative