Meanwhile my Open GL routines can emulate most of the old graphics engine functionality. One remaining problem is the so called "day and night" mode that uses trickery with color maps to make certain colors light up at night. I have no proper emulation for that yet.
The other may or may not be a problem, the old graphics engine uses the same palette changing technique to show vehicles in different colors depending on their owner in the game. Many graphics sets even before my changes did not use that features and preferred vehicles in natural/realistic colors. Model railroad fans usually want the original liveries also. If requested, I can emulate this to some extent, but I'll wait for feedback on that point.
Besides porting the graphics engine to Open GL, I did similar for audio, and there is now an audio system using Open AL. Pretty new, little tested as I write this, but on my two test systems (Linux Mint 21 and Windows 10), it works.
UI changes are going slow, after the start screen to select a graphics set, I've tried to improve the "new map" dialog some, aligned buttons and normalized sizes. The real tough dialogs are still to do, the vehicle depot and the schedule input dialogs each have dozens of options and little guidance for the player how to use them.