Driving Motor Vehicles in Trainz?

I see on the front page to Trainz2012 it says

"Drive Steam, Diesel and Electric Trains (and even trams, boats and cars)"

Driving trainz seems fairly straightforward, but how would one drive a car? Is it something you have to download?

It's about "type".

You can drive anything that appears as a "traincar" and contains an engine spec. You can modify existing assets to work this way. You can drive it on regular track, or lay invisible track and place it on that. I may wind up doing this with trucks going to and from specific businesses. Not enough time to explain fully right now, perhaps someone else can chime in?

Carz and boatz and planez and blimpz and shipz and even wagonz and horse teamz can be assigned to invisible track, given invisible speed limits and invisible junctions and can be driven as if they were locomotives.