Can I make my nether tunnel work in unloaded chunks?
You can use detector rails (archive).
Replace a normal rail with it, take the redstone signal from it and use that to transform the next part of the tunnel into minecart mode. Then, after a short delay (long enough to let you drive through), transform it back.
Apart from that, is there any advantage in having the tunnel in minecart mode? How I see it, you could just permanently leave it in ice road mode, it can still be used with a minecart.