No direct experience with Metro DC. This is always a "touchy" topic - here in NYC we "beat back" a proposal to ban photography in the subways when they tried.
Originally Posted by DBP
Setting up a 5x7 is more problemattic than the usual 35mm "street" shot. I'd suggest an inquiry to the Authority (e.g. Public Affairs officer) detailing what, where and how and asking if a permit is required for a "commercial-type"* shoot.
Besides the usual "terror" concerns, they are also going to be concerned that you don't interfere with passengers, crews etc. Treating like a "commercial situation" may actually get you what you want....