I'm sure there's a good reason why there's not more nether portal style train interaction, but I'm not aware of the reason. I get that you'd need to actually tell the train tracks what the big idea about putting obsidian in front of the train is about so they can put the train through the portal, but why doesn't that work for other types of portals? Most of them work by simply teleporting entities I understand, which would derail the train, but surely it's not that hard to copy the nether portal code and remix it for a different portal? Even Create:Teleporters to my knowledge eschews track continuity. But whenever I ask about it people clam up or say it wouldn't ever be worth it, which I can respect since I'm ignorant, but without solving that ignorance I'm left just imagining how cool it would be to access more dimensions via trains and have funkier nonlinear train lines.
Imagine: the sharpest possible 90° turn in a train is 9 blocks by 9 blocks, practically a decameter or more, plus the train loses speed as it turns. Portals however can be placed wherever you want and thus can be placed at 90° in a comparably microscopic amount of space, and the train wouldn't ever have to actually turn! Currently the train must enter the nether, which requires a lot of security and terraformation and building to not look extremely out of place, and still might struggle to avoid the required turn if portal placement doesn't cooperate. The nether's 1:8 block ratio is at times useful for shortening train routes, but usually that will be an additional obstacle that makes the nether less appealing, not more.
