Arrives Glenfinnan 1054 (and don't forget there's an afternoon train as well at 1518). It will cross the viaduct a few mins before this. Timings here.
Are you aiming for a photograph or just to view it? Most people seem to form a gallery at the edge of the visitor centre car park but for a better view I recommend walking under the viaduct then immediately up the hill to your left so that you're looking down onto the oncoming train. Parking is free in the other (slightly concealed) car park - drive past the main visitor centre, over the river bridge, then it's immediately on your right. Then walk down that path to get under the viaduct.
The other good viewpoint is up behind the visitor centre - simply follow the footpath up the hill next to the entrance to the visitor centre.
Photo below - I got to the viewpoint in about 40 minutes from Fort William never having done it before so it is possible!