There are lots of interesting abandoned parts of the railways in the UK. Here is a few of them which I have explored.
The abandoned Highgate station is directly above the underground station. The station is on the abandoned part of the northern line called the heights. It goes from the northern city line (Findsbry Park to Moorgate) and connected it up to the northern line at East Finchley. There also used to be other parts of the route that went to Alexander Palace, and anouther part of the route connected the 2 parts of the northern line from Mill Hill East to Edgeware.
Abandoned railway in Gillingham
This railway comes of the main line east of Gillingham station, by the level crossing (joins facing the coast). It then goes in a large cutting down towards the river, over a bridge over the new road, then that is now where it finishes. It used to go into Gillingham docks (which has now been demolish for new houses to be built on the site). In the 80s the railway went even further than this, it connected up with the tramway at Chatham Dockyard. This railway was last used it the 90s when the nucular waste that had been dumped on St Mary's island was being cleared so that they could build houses there.
Lydd Town railway station is on the Dungeness nuclear waste line. This branch line comes of the Ashford to Hastings line. Today the line is only used occasionally to remove nuclear waste from the power station. Originally the line used to go to New Romney as well as Dungeness. The track bed (still with southern's Exmouth concrete fence posts) can still seen running across the stony wasteland parallel to the Romney hythe and Dymchurch railway (a minimum gauge line which is open to the public).