So on Monday I went to Yorkshire on holiday and passed Ratcliffe power station, as seen on the MML route in TSW. However I realised Toton Depot was nearby, so I would eventually last night return home. I looked to find it on tsw5 just to find this:
It still kinda is try it anywhere else and it'll tell you that you are outside the playable area and it'll force you out the game.
If you want to get behind the invisible walls with a higher vantage point, there are routes that have a glitch that allows your camera to fly through the whole map without any restrictions. As a passenger jump on a train that has a portal as the next stop. Sit down. Switch to external camera. Wait for the train to stop. These pictures are from SEHS using a London bound train from Dartford and WCML using a 377 heading to Croydon.