Quoting my bug report from DC:

1. stock isn't "processed" when its out of players' render distance
   which just causes the train to stop as soon as it runs away from render distance, im guessing that's because it doesnt load CC chunks
2. after my train consisting of loco and 8 carts ran away from render distance and i went there to see where did it go - all 8 minecarts got "compressed" into 3 in a way that actually there were couple minecarts in the same spot (last time i thaught they disappeared but fortunately they didnt, their positions were just messed up)
   Other than that - everything seems to work correctly

when I tried just placing tracks then saving then playing the same save the game crashes

Can you post the crash?

it appears to still work on normal cubic chunks
but if I use the cubic chunks save converter it crashes here is the crash file:
crash-2024-06-25_02.28.44-server.txt

That's a crash in cubic chunks, they will need to fix that crash.

yep can confirm this happens