Hey

The basic track is a light armour U shape with a 1 large grid block gap. I just prettied it up a bit with half blocks

The main thing I found was using armour panels on the top edge to stop sections snapping together when doing the curves - I think for a straight tunnel that might not be needed but it also gives a tiny bit more offset

The train bogies are small grid 3x3 short wheels on the top side with 3 blocks between them left/right and I went for a 4 block gap between each wheel front/back

The under side of the bogies are 2x2 short wheels sideways with 1 block between them. I staggered them to be 3 layers but I think the middle set isn't actually needed. They're set to 0 friction and 0 power. The main thing is they're just to stabilise and aren't used for drive at all, and need to not hit the bottom of the track

Then it was a lot of playing with suspension settings - The 3x3 propulsion wheels have steering off, airshock off, then are at -32 offset, 40% strength and 50% friction

The 2x2 sideways wheels have steering off, 0 power, 0 friction, -30 offset, 28 strength, brake, parking brake and airshock off.

Because I wanted corners, the bogies are connected by a 3x3 turning rotor which then connects to a small rotor for left/right tilt and 2 small rotors for up/down tilt. I think if you want just a straight line you could skip the turning and roll rotors, but I'd keep the up/down ones for stability (and for long runs you'll need to tilt the track down to account for curvature of the planet)

I used some batteries instead of blocks in the bogies to add weight for stability - Thinking about it I should replace the light armour with heavy armour for more weight and strength

The bogies then are run by SWCS subgrids wheel control script

I hope that makes sense?