Vehicle Mod Help - Wheels wont connect to frame

[craggy sapphire]

I'm trying to make a proof of concept locomotive, but the wheels wont stay attached to the vehicle and I have no clue what I'm doing wrong. Here are some pictures if that helps.

[left haven]

the first picture says that your issue comes from the wheel having multiple shapes

second pic doesnt say anything, useless

third pic says your wheel is probably set to prop=true