Had a reply from DTG support:
"To rule out a conflict with your files please run a Steam file verification: https://dovetailgames.kayako.com/Knowle ... game-files
After doing so please open the game and enter the settings. From here select "Tools" and then the option to clear your cache.
Once you have done so the game will need to restart. Once it has restarted open the "Standard" menu and select the search icon in the top right and type in Colton.
You should then see all 21 scenarios included with the route as depicted in the attached image. Please note that they are listed in such a way that 2-9 appear underneath 17 so you will need to scroll down to access them."
Rather frustrating that the Steam versions need this work-around to make the scenarios visible. The originals purchased directly from All Aboard I can understand. They are after all a bit old. But I would have thought DTG should have fixed the problem when they were added to Steam.
I have now requested refunds from Steam for all three routes and re-installed the originals. The work-around works on them too. (And I have more scenarios!)
Bob.