When I was still in Pittsburgh 2 years ago, the bridge you refer to (known locally as East Conway) was off-limits to railfans. Extremely sad, as that was a killer place to shoot from.
Your best bets are going to be trying to shoot from the parking lots of the businesses that line Route 65. If you're feeling frisky, you could dash across the road and shoot from the shoulder, but if I recall correctly, there isn't much of a shoulder and you'll be taking your life into your hands.
Sure, you can drive down into the yard. But if you do, you'll likely end up making a small contribution towards NS' next quarterly profit summary...this ain't CR country anymore.
The because of the geography and hostility from NS, I rarely did any shooting of the yard itself. But if you want to see a buttload of trains, spend a day hanging out at Rochester or Leetsdale on either side of the yard.