Note: I have not used an existing 3D engine I played around with a few,all were to limiting or to slow.Same thing with SDL the 3D part of these is chronically slow although the sound appears to be OK.
Note 2: Distant terrain is a MUST.... Very difficult to do along with the rest of the scenery..
Fortunately hardware has improved quite a bit in the past few years. In early 2006 I played around with Ogre3D a bit with the intention of maybe trying something train-sim related as it was just MSTS (for me) at that time and it had a horrible habit of bluescreening anything in its path. The speed definitely seemed at least adequate although having very little game design experience, the bulk of time would've been spent by me trying to figure things out.
OpenBVE though seems to have gotten things right, it easily gets 75 fps here at 1280x1024 with all filtering/AA settings on max. And I must say I'm impressed by the degree of realism, from the way fog seems to surround nearby objects, to lighting, to train movement, the appearance of track, and of course the incredibly accurate sounds as the train rounds curves and passes over points (both are very important to me).
Distant terrain is very important too.. I hope this is improved upon in RS with Mk3. RS already comes with good scripting abilities in the audio department so realistic sound is already possible with the right samples and scripting. Now we just need the dispatcher fixed and some cab sway
It's definitely very exciting to see the progress openBVE has been making though. Especially the vids posted here.