For those who are wondering how to carry your trike on your car...here's one way using a Saris Bones 3

How is the rear wheel held in place?

The rear wheel justs rest on the car. There is no need to secure the rear wheel as it doesn't move. The bike rack secures on the T-shaped cross bar which is rigid. The pivot point where the front wheel turns is near the hubs.

I would think there would be a lot of scenarios where the back wheel could slide sideways and rip the trike right of the rack. There is a reason that all contact points are secured. Straps to restrict side to side movement of the back wheel are required. Terra Trike has a new trike carrier for $100.

There is no way for the rear wheel to move sideways because the T section that is secured on the Saris Bones is rigid and doesn't pivot. Also driving speed while carrying the trike is not fast around corners so there is no danger of the trike coming off the rack.

Here's another option (review here): Terratrike's Universal Trike Carrier ($100)

glad to see I'm not the only one carrying a trike this way...I just bought a 700 a few weeks ago and dug my 18 year old Saris Bones Rack out of the garage to haul it...works great though I would recommend tying down the back wheel via a strap run thru the back windows and inside of the car for speeds over 55 mph...I don't think I would want to take it out on the interstate at 70 mph or better without doing so...