I live in the Richmond VA area, and I'd have to say the best place to railfan around here is the town of Ashland about 15 miles north of Richmond, just off Rte. 95.
It's on the old Richmond, Fredericksburg and Potomac two track main line, now owned and used by CSX and Amtrak. Usually there's quite a bit of train action, at least one every 45 minutes to an hour. I say usually because while the Amtraks are fairly predictable the CSX freights aren't.
The old RF&P station still stands in town, it's a visitor center now (with restrooms!) and has a small RF&P museum on premises. Plenty of amenities in town with places to get a meal or a snack. AND just down the street is Tiny Tim's Trains and Toys, a nice model railroad shop should you get the fever real bad. They sell railbooks, magazines, and videos as well.
No bridges or tunnels in that area, but the train action makes up for the lack of same. And you can get close enough to the trains to touch 'em, not that you should!
As far as bridges, Richmond has the old C&O James River trestle, at about two miles long it's the only river trestle in the world that gets it's feet wet without actually crossing a river! Built in 1901 it still serves CSX freights and coal trains but the train frequency is unpredictable, you can go hours without seeing anything. One interesting thing is you can see the old Civil War era trestle piers still in the river, however without spans or trackage now. Oh, aside from a Porta-John there's no amenities in the James River Park parking lot by the trestle.
Hope this was a help to you.