Well I live in the DC metropolitan area, so I'm pretty familiar with the tracks around there. The edge of Benning Yard is adjacent to the DC metro's orange line at Minesoda Avenue NE station, a pretty bad area, if you stay around the station during the day you should be okay but I wouldn't flash a good camera around. You can walk to Benning from the Benning Rd station on the blue line as well. I've never been there myself, only seen it from the metro, but I have been to the yards and layups a little before Benning at the border between DC and PG County Maryland, also adjacent to the metro orange line. This is where they leave many scrap gondolas and sometimes coal freights going to benning and occassionally even boxcars, there are active tracks leading into a scrap yard as well (the reason for the gondolas). Between Cheverly and Deanwood stations, about 100 feet after the scrap yard is a little lot behind a business where many drug addicts hang out (probably the "shanty town" mentioned above), they're more interested in scrap metal than anything else though. Theres a lot of homeless (often drug addicted) people in that area, but none have ever done more than ask for a cigerette from me.
There is another nearby freight yard (or gravel loading station) in Bladensburg, MD that sends freights to West Virginia almost daily. They may let you take pictures near it, I've never tried to go into the yard itself because it is a bussiness. That is on a side road off of Kenilworth Avenue, near the Parkway leading into DC. It is maybe 1/3 of a mile on the tracks before the new csx bridge construction also in Bladensburg.
