David, as a young lad growing up in northwestern Pennsylvania, I have memories of two fields where I played Little League baseball that were adjacent to railroad tracks. One was in Akeley, PA next to the New York Central's Dunkirk, Allegheny Valley and Pittsburgh line. I still remember seeing a northbound freight consisting of one GP-7, boxcar and caboose passing the field as my team (the Sugar Grove Merchants) was locked in an epic struggle with the Pine Grove Lions. IIRC, it was one of the only games we won all year, however my memory may have faded in the ensuing 40+ years-although I still remember the train clearly.
Another field where the railroad tracks bordered right field was in Clarendon, PA where the Pennsylvania (now Allegheny and Eastern) ran between Warren and Kane. The field in Akeley allowed us to watch the train pass while on the playing field, to watch trains in Clarendon you had to turn around, taking your eyes off the action. I still pass this field when I make my yearly visit to my parents-it is now weed grown and neglected, making me suspect that either the league has found better accomodations or that the youth of the area are too busy playing Nintendo or Game Boy to become involved in Little league baseball.
New York City's Yankee Stadium has a constant parade of subway trains passing in right field. Baltimore's Camden Yards Stadium has both light rail and a CSX line running right outside the stadium. The view of passing LRVs (how I HATE that term-what's wrong with trolleys?) is blocked by the old B&O warehouse. Trains to Oriole games ended a couple of years ago and I have no idea if these tracks see any service.
I know of at least one minor league stadium in the southeast, although which one escapes me at the moment, where a busy line passes along the right field wall.
Interesting topic! I hope we'll see more responses!