It seems the setting that disables physics collisions on bricks doesn't always work, and I have no idea what specific shape causes it. I tried doing a similar shaped build to try and see what might cause it and reproduce the issue, but to no avail. (I didn't build this train so I do not want to distribute it here, but I can DM it to a dev if needed)
What's weirder is that sometimes if you copy and paste the whole grid while the collisions are "broken", it just magically fixes itself.
Happens both in a listen server and a dedicated server.
#Physics hulls still considering bricks with collisions disabled (in specific situations)
tulip zephyr
mighty flame
simplified collision is not generated with unlimited detail
that brick is likely too small to be in a high enough octobvh node for it to matter at the level of detail considered by the convex generator
to make it hidden from the collision you will have to find all other bricks in the same node and disable them too