I started the game on 20th August and it only comes up with the 3 standard routes not the Southeastern high speed route I purchased in February when it came out. I've also lost all the mods as well for this route as well as the mods for the Bakerloo line. If anyone out there could help me with this, I would be extremely grateful! I've attached some screenshots as to what has happened.
Verify integrity of game files on Steam. I deleted everything from my DLC folder forgetting that actual DLCs are there too, not just mods. So when I loaded into the game I only had the 3 base routes. This method got all my DLCs back
If the verify integrity doesn't work. You could try in steam going to manage DLC on the TSW2 Page. Then unticking Southeastern High Speed, give it a few minutes to let it think it's uninstalled. Then tick it again and if it's worked, you should see it download. I realised after many hours last night that Arose was missing for me and that worked to get it back.