(Note: Grade Separation in quotes because it's not necessarily actual grade separation - just the illusion of such)

When watching the train launch pads, it came to mind that although Factorio was patched to fix the workaround that allowed for https://mods.factorio.com/mod/rail-bridge to work, the way Renai works could actually be 'adapted' (haven't read the source code; am unsure of the feasibility) to support actual railway bridges (or at least the illusion of such). Although I have not tried out the mod myself, my understanding is that as long as there is a launch pad and a track at the landing site, the mod allows for trains to pass in a grade separated manner (albeit currently with one train flying over the other).

It therefore came to mind that if you could force start and end points for the flying, all you'd need are placeholder graphics for the 'bridge' (and adjustment of the train graphics in midair) and we'd suddenly have actual train bridges. Of course mod compatibility would be a wreck and there would be a lot of edge cases, but if used for the specific purpose, it might actually provide a vanilla-ish method for grade separating train lines.

Just a thought; will definitely try out this mod on my next Factorio map regardless since it seems like a lot of fun.

Definitely a possibility, I can imagine how a bridge like that would work in my head which is usually a good sign I could pull it off given enough time

Good to hear! Though of course handling speed of the vehicles (and edge cases such as running out of fuel mid-jump; player overriding orders mid-jump) might prove challenging.

After a lot of thought, I could never actually come up with a tunnel design that 1. would look good and 2. be something I could actually make. But i know the speed finikyness is something people don't like so the middle ground I came up with was a train ramp that uses electromagnets to guide jumping trains to an exact landing point regardless of speed, with the "downside" that it can't do jumps as far. I know this post is a month old now but just thought I'd comment here in case.

Cool! Hopefully something similar eventually makes it to vanilla (we can wish) but it's great to see more iterations of the idea.

instead of a train tunnel, a raised bridge would also work.

Looking forwards to testing out the new magnet train ramp functionality once irl burnout has ended :)