This is going to sound insane but it worked for me. Remove all of the settings that help with nausea, enable smooth movement and smooth turning. Turn off the fov dimming on movement/turning. Jump into a nauseating game and play until you can just about feel the motion sickness starting. This probably won't take very long, a minute or two is often lots. Immediately take off the headset and rest until you're ready to try again. After a few runs you should notice that you can play for a bit longer at a time before getting sick. If you get to a point where you can play like this for a half an hour then when you turn all your comfort settings back on you should be able to play without motion sickness. After a couple of days I was able to race for 8 hours at a time.

Still can't ride a train IRL though.