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From M25/A1, South Mimms services:
Follow signs to Royal Vetinary College. Some of the roads you need to go down are single width but dont be put off. When you get to the College main gate drive straight past for about 200 or 300 hundred yards until you come to the rail bridge. From here follow the footpath on the west side of the line, and find a place opposite an OLE mast.

The is an area big enough for a couple of cars just before the bridge.

Exit the Station on the west side and make a U turn onto the footpath, then follw the path south to the location.

Anglia Gen

Update April 2018:
This location now requires steps as the line side has been fenced off with palisade fencing.

The favoured shot at the location is of northbound workings from the west side of the line, so only time you'll get good sun light is late afternoon in the summer. But the location is quite open and trains will be passing at speed.
Noise wise you are quite a distance away from any traffic so video recordings should be fine.

Brookmans Park Station
Hadley Wood Road Footbridge
Hatfield Station
Potters Bar
Sopers Viaduct, Cuffley
Welham Green