Getting my TSC install back up to speed, installing the Kyle Line which I originally purchased direct from Just Trains. However the route has since appeared on Steam so was wondering if scenarios created in the JT version are compatible with the Steam version including the ability to upload to Steam Workshop. (Thinking Bossman Black Five hauled trains...)
Yes, they should be as the route GUID is identical. The only difference is probably that the JT version comes with more scenarios, and Steam distributes them in single .ap files whereas JT files are not packed.
While this is not a compatibility thing, I just wanted to add that JT sometimes add some goodies. I've bought the Cornish Main Line directly from JT, they've added a second tracks.bin and a little tool to switch between standard tracks (3d rails, 2d sleepers - good performance) and 3D tracks (3d rails, bolts, sleepers).