When a new locomotive is placed and let run at speed 4 in a small loop the model "jumps" up while it is turning. This stops happening once the world is reloaded.

It seems to happen from 9.10 through 9.12.2

Here's a gif of it happening: http://imgur.com/mtpKqTJ

It consistently happens for me, but I have no idea if it's reproducible for anyone else.

This seems to be fixed in 1.10.

Meh, this is Minecraft being stupid again. Something about improper syncing of some variable or something in the base entity code.

Ah, I've never seen it before

I've seen it while messing with the tunnel bore.