Allan sent me this link, from the “Train Fanatics” site.  I’m not sure such a rail tour would work in the UK – imagine the problems on the London commuter lines – and I guess all our lines are just too crowded.  The article says that…..

 A massively popular hobby among rail enthusiasts, organized clubs exist across the United States and Australia providing organized, safe and legal ways to travel the rails on these wild machines!

It is not uncommon to have 40 or more cars lined up on excursions that can last for several days, covering hundreds of miles. All participants travel in a group with the host railroads permission for the use of the tracks. Crossing guards are used at intersections since the speeders do not have the right of way over vehicles.

The rail speeder, otherwise known as a motor car, was invented as far back as 1896 when a vehicle was needed to help the maintenance of way crews and track inspectors to move quickly to and from work sites. Early speeders were human powered pump types later followed by gasoline powered engines.
These types of speeders were later replaced by pickup trucks with flanges to allow them to hoist themselves onto and off of the tracks. The speeders left from the earlier days are now owned by hobbyists and can cost several thousands of dollars to rebuild and maintain.

And here’s a link to the YouTube video….