If you add sockets to you carriage mesh (you can do it in ue4 in mesh manager, just open your mesh asset and socket manager will be at the bottom right corner) then you can constraint your wheels directly to sockets. Axle was necessary just to be able to turn wheels on the truck by turning axle, carriage doesn’t really need them.
When you constraint wheels make sure to allow them to rotate freely in Y axis. I don’t do that because with no-friction physics material wheels are just sliding over the surface.