I'm playing with biter expansion on, and invulnerable railways because I wanted a challenge, but to spend half my time repairing train tracks.

But it seems that creep expansion has the effect of removing my train lines when they set up next door? I built a long distance outpost weaving between some bases, and after a while, my trains couldn't path any more because some bases had expanded and damaged my tracks.

Is this meant to happen, has this already been fixed or is there anything I can change to not let this happen again

"Creep expansion"? I have not noticed the creep expanding. Does it? I know new creep appears when a spawner spawns. Are you sure a spawner didn't spawn on your tracks? (Or right next to them?)

Yeah, biter expansion next to the tracks made the creep grow and remove the tracks (not creep just growing on its own (although that could be interesting if you had to fight it back with flamethrowers))