In BVE2 and BVE4 you can not simply "pick" the train you want to drive like you can in OpenBVE, you have to have exactly the train that the route suggests (It depends on what route you want to drive) You can figure out what train the route wants you to drive by opening the CSV and looking for where it says "With train folder". It should say the name of the train after that. So you either have to look for that train on the internet and download it, OR you can rename any trains you already have to what the route wants you to drive.