I bought RRT2 Platinum two months ago and blazed through the entire classic campaign. Now I try to play the single scenarios and I´ve run against what I think are some bugs:
- Passenger trains won´t load if the station is serviced by more than two trains (even with a passenger output of 0.5 per year).
- Some missions won´t have the "win/defeat screens" I.E, I had the silver and bronze victory conditions for Pennsylvania and Viva Mexico!, but the win screen didn´t show up on the first one and the second one told me I had lost.

What can I do to solve this?
Thank you very much.

I can't comment on the problems with the passenger trains, never had sth linke this...

Regarding the missions, yes, some are broken or have incorrect descpriptions. You can try to look for fixes for them here:

http://theterminal.dune2k.com/?p=rrt2-files-maps

That´s disappointing, I thought with the game in a virtual platform some of the scenario bugs would be fixed. The Viva Mexico! one goes back to when I had the CD version.
Regarding the passengers, it only happens to me, the AI runs normally.

How do you download the scenarios and make them playable for this RRT2 version?

Search within the three "Fixes"-Folders for the map you want (e.g. Viva Mexico in Fixes-Gold), download from the button and extract the archive to your RT2/maps folder. I don't have the gog version of RT2, but it should work for all.

All GOG tend to do is fix the installer to run on more modern operating systems. The original developers will have long since disappeared and official support ended years ago.