Also, if you go north along rte 15 I think, it is not too far to Duncannon. The station there is restored as a business and you can acess the tracks from the platform to take some photos and use the statin as a photo prop. It is a good place to get the freights moving fast along the river.
I can also reccommend the Bridgeview B&B. I have stayed there a few times and it is a great location to watch trains on the Rockville bridge. If you don't stay there, it is not polite to go onto his property or deck to watch trains. However, if you continue north a few 100 yards along that road, you get to the north side of the Rockville bridge where there is a large pull off area that has open veiws of the bridge. You are a little lower than at the B&B but can still see the trains going across the bridge quite well.