The combination of such a light-weight (relatively) vehicle with a lightely used branch line poses one serious problem -- activation of grade crossings.

Lightly used lines or lines that have been completely dormant suffer from rust buildup that interferes with making good connections through track circuits. And light vehicles already have problems completing track circuits. Remember that a hi-rail vehicle typically uses the flanged wheels only for guidance, and most of the weight is still on the rubber tires, which are used for propulsion. (Hi-rail maintenance vehicles cannot count on grade crossing equipment activating. They have to stop short of grade crossings.)

I think this is one of those things that keeps being re-invented -- until people realize the practical problems in using such a "solution."

-- Ernest