In the summer the vegetation can get a bit thick around the edge of the sidings for the footpath near the river. If you continue away from Ely, cross the footbridge over the river and walking away from Ely for a quarter of a mile, there is the access bridge for lorries into the site. I think that is private, but when its quiet, you can see a through the gates
https://goo.gl/maps/k3bXz1beJK9KBUYo7
That road isn't the nicest to walk along, with a rough grass verge, and fast traffic.
If you wanted to walk to Queen Adelaide on the East side of the river, walk 200m South of the footbridge. There is then a nice track, which is part of the Hereward Way. After half a mile, you can turn left onto a path that heads to Queen Adelaide. Head back towards Ely, cross over two level crossings. Then there is a footpath just the other side of the trackside fence. You can then see the sidings at the front. The path continues, with a foot crossing across the line to Peterborough, and another across the loop. You then end up at the Northern end of Roswell pits, by the sailing club.