I think Transits go by rail as well as road out of the Western Docks...there is a loading platfom on the siding off the Western Docks branch. The vans and chassis-cabs are driven in convoys from Swaythling, followed by a minibis that then takes the drivers back. It would appear that imported Focus cars come in by jumbo transporter ship and they then get stored in a huge compond the docks side of Millbrook station. We see Land-Rovers come down the M271 to the docks on transporters as well as Hondas ex-Swindon and MINIS ex-Cowlet (sorry BMW Oxford). for export.
The car's come into the Western Dock's as well, off the ships and there are, at least, two huge multi-story car parks and a bit of a ground level storage.
I like looking at all the shiny new car's as I walk through to get my boat.
There is a couple of out-of-use ramp's to load the trains with cars/vans etc at Millbrook station (other side of fence/docks side) which I thought were Motorail, but was unsure, more likely used to be the main hub for transporting cars, before the new facilities were built at the Western docks and before the car factory (no idea of name) was demolished a few years ago.