Two weeks ago, Steam introduced a "bugfix" which unfortunately broke Simutrans. I was forced to introduce the simutrans.sh file to load libraries from the simutrans directory - this is a thing the preloader needs, so yes it is specific to the Steam installation.
I created a new depot, which includes the .sh file an the necessary libraries. Can you tell me if you also lack the "lib" directory in Simutrans Steam folder? I've reinstalled the game, and Steam does download me both the .sh file and the lib folder.