I accidentally found an infinite steel/concrete generator that I can do whenever I want (I’m not using it but like it’s a reproduceable bug). If you drag to place a road or train track, but don’t click on it’s end location to confirm, you can go down the the delete button and try to delete the road/track preview and it will give you 50% of the building cost back. Except you never had to build the road/track in the first place.
Here is a video showing me doing it:
Google Drive
creative mode light !