[quintushoratius]

> It would probably require electrically driven thrust carriages rather than pneumatic - you would need one for every car using the system.

Not necessarily one per car. You'd probably have a chain of thrust carriages, with maybe some spacing in between each individual carriage. One or several cars would clump onto a thrust carriage -- there's nothing to prevent them from being quite long.

When it's time to get off the highway there might be a hole in the clump for a bit until someone getting onto the highway fills it in.

The one issue I see is that, in order to disconnect from the thrust carriage, you have to have a way to do so. The logical method is to use an electromagnet that can be powered on/off to connect/disconnect but then the car has to power that -- and that might sap it's range a bit.