Since I started talking about Rail Trucking, a number of helpful souls have suggested that the idea has been done before. These suggestions haver included hi-rail 4WDs and trucks and RoadRailers.
This example is from a Tasmanian scenic tour company that uses some rail line is the centrepiece of their tours.
Below are some example from the Wikipedia section on Road Rail Vehicles.
Why Railtrucking is better: Besides the fact that the design seems to be geared to short distance slow speed rail use, what kills this approach is the fact that the bogey wheels are permanently attached to the vehicle and a considerable amount of weight and inefficiency is wrapped up in the hydraulics that lift the vehicle up once it is over the rails. With Railtrucking the bogey is left trackside for others to use. The weight of the bogey does not effect the fuel efficiency of the vehicle when it is used on the road.
Additionally, while it seems at first blush to be a great idea to have the car tires drive the vehicle while on the rails, this would quickly make the tire unusable when used for any distance as a wear line would make tire wear uneven and dangerous. With RailTrucking the whole tread of the drive tire is used for driving and braking.
Roadrailers are semi truck trailers that are fitted with rail bogeys so that they can be drawn along in a normal engine driven train
Why Railtrucking is better: Roadrailers underscore one of the main reasons that the system has to avoid the traditional reliance on railway carriage coordination. With Roadrailers, you still need for the whole train to stop at its destination and then coordinate decoupling and disconnecting of the Roadrail enabled trailers before you can connect it to a semi and get on with the road based element of goods delivery. The benefit is mildly better than having a container shipped on a train flat car.
With RailTrucking the whole truck travels the rail and the truck driver can determine when they leave the rail. The drive-on, drive-off design of the rail bogeys mean that the truck spends almost no time transitioning from road-based to rail-enabled and back.