Toyota and subsidiary Hino Motors are joining forces with Japan Rail Hokkaido to develop vehicles that can drive on roads and railways. The DMV (Dual Mode Vehicle) has rubber tires and steel wheels, and can convert in about 15 seconds. They began testing the vehicles 18 months ago.
Combing the versatility of a bus with the speed of a train has allowed Japan Rail to tailor routes and services to the communities it serves. Rather than scuttling service on under-utilized lines, Japan Rail has simply switched to smaller vehicles.
Dual-mode vehicles would be a great addition to America's mass transit infrastructure. It would make rail transit feasible in those areas that don't have the population density to support a lot of stations, and make mass transit a more viable option for exurbs. Riders could simply hop on at a bus stop in their neighborhood, then ride the rails to their destination.
The new vehicles also use a lot less diesel fuel than conventional busses. http://blog.wired.com/cars/2008/05/half-bus-half-t.html
(image credit: NeiTech)
Ask Amtrak, it's a large part of the reason they habitually run late.
Plus it requires literally millions of dollars in liability insurance to put a private car on the tracks.