I am only asking this as I am new to the code of Factorio:
I have seen a Mod that turns off collision with pipes so you can walk over them, is it actually also possible to just turn off collision with trains, or is it not, hence why you set up the mod this way? Not questioning decisions, just wondering.

you could remove collision boxes from trains, quite how the game would behave in this scenario I have no idea.
I did the mod using the rail entities as I don't like maths so didn't take the collision box extrapolation route and I found the less you mess with the core game mechanics the safer and more compatible the end mod should be.