There's plenty of decent photo places, e.g. Ely station itself, then walk north along the riverside to Kiln Lane (which if you turn left crosses the lines from Ely just south of Ely North Junction). One of the classic 'Ely' shots is from the riverside looking towards the cathedral so you get the train with cathedral in the background. The B1382 road (Prickwillow/Ely Road) crosses all of the lines just north of North Junction. There is also a public footpath south from Queen Adelaide which crosses the Ely - Peterborough line just north of the junction and also crosses the 'West Curve' loop line (which is used by freight traffic to/from Middleton Towers sand quarry near Kings Lynn - and the Norwich direction - to directly access the Peterborough line without having to reverse at Ely station). Have a browse around
https://www.openstreetmap.org/#map=16/52.40672/0.29251 , Google maps/streetview etc.
Also, if walking north along the riverside, where the riverside path veers right and goes under the railway, if you veer left instead and go through the small Fishermans car park, there is a path which takes you up a small embankment to a couple of benches with a view of the railway, with the river and Fenland in the background. Not especially good for photos due to restricted viewing angles, but fine for just watching the trains go by when the weather is decent. See
https://www.openstreetmap.org/#map=18/52.397357/0.275935
(
Note - all the lines around Ely are busy, and many of the tracks are bi-directional, so if you are using any footpath crossings in the area always look very carefully both ways before you cross the tracks and don't hang around on the railway side of fence).