This 21 span viaduct, one of the first in the world to be built from concrete in 1901 carries the Fort William - Mallaig steam train, used as the 'Hogwarts Express' in Harry Potter movies. If you calculate carefully the train times you can time your visit to see the train cross the viaduct (hint: it takes about 30 mins to reach Glenfinnan from Fort William, and about an hour and a quarter from Mallaig - we worked out that we could see one steam train heading west at 3.15 and another heading east about 10 minutes later! But times may differ next year of course so do check.)
If you have decent boots and fancy a bit of a climb to a superb view, do not park at the visitor centre, but about 50 metres further along the road (towards Glenfinnan village) on the same side at a nondescript small car park. Walk out of the back of the car park along a tarmacked track for about 10 minutes until you reach the foot of the viaduct. Cross the river at the small bridge just before the viaduct, go under its arches, and take the footpath alongside the viaduct that climbs away from the river. After just a few minutes' climb (if you reach a wider clearing in the path you've gone about 25 metres too far), when you see the rocky outcrops to your left find a suitable spot to turn and yomp across the heather towards them. You only need to go about 50 metres to have a stunning view of the full viaduct with the loch behind - perfect for photographing the Hogwarts Express! Warning - the heather is almost always wet, so if you do not have appropriate footwear be prepared for everything below the knee to get soaked!