I bought several routes in TSW2, and a couple of locos, in a sale specifically because they compliment each other by adding layers/extra services to other routes. Does anyone know whether the TSW3 versions of these routes will retain those layers and work the same as they did in TSW2?
The TSW2 DLC should be exactly the same as they are in TSW2, layers and all. The only old DLC which has a specific "TSW3 version" is SEHS, which has a whole new timetable and set of layers to go with its extension - in particular the diesel railtours have been replaced by steam ones.
Anything that was playable in TSW2 with layers will still have the layers on when they become available on TSW3 as part of the TSW compatible system. Only SEHS extended has changed and does not have the diesel railtours in and the original version of SEHS will not be playable in TSW3. The TSW compatible routes (legacy) will be the same as they were in TSW2 with the exception of routes having the dynamic weather feature.