I hate to be cynical, Peter, but #30's arrival in DC can be hit-and-miss. I've caught this train 5 times into WAS, and we averaged close to an hour and a half late each time. That said, the last time we came in on #30, we were only 15 minutes down, so it can happen.
Assuming you come in with good time, I'd think it would take you upwards of half an hour or more to do what you need to do to take any photos there. From there, Union Station is actually a destination in itself in Washington, DC. Lots of people actually go to this train station without any intention of catching a train - reason being the mall, food court, restaurants, and movies.
Even if the station doesn't captivate you, I probably wouldn't suggest wandering too far. DC is a very easy city to lose your sense of direction in, despite the easy "letters and numbers" setup. One quick suggestion from a rail perspective would be to follow the signs to the METRO, catch a RED Line train for Glenmont, and ride to the New York Avenue stop (next stop), then chill out on the platform for a couple minutes to view the New York Avenue yard before catching the Shade Grove bound train back towards the Station.
If you've never caught the Capitol Limited before I would tend to suggest focusing on the right side of the train from a comfy seat in the Sightseeing Lounge. Daylight should come just after Pittsburgh, and you'll have hours of scenery passing you by with the rugged Youghioehenny (sp) River east of Pittsburgh leading you up to "Sand Patch" which is the height of the Alleghenies.
From here, you'll be then descending for the rest of the ride, probably the best part of which comes shortly after leaving Cumberland. There is a HUMP yard on the right where freight cars are left to free roll into a yard for classification and coupling. After this comes some really good mix of bridges and tunnels as you ride along the border of West Virginia.
If you're interested in stuff to look out for along the southern part of the NEC, just let me know.