Actually, weight is/was a major factor; as lighter vehicles have more problems shunting the track reliably. Compounding that for hi-rail vehicles and many other track machines is that much of the available weight is diverted to rubber tires or to various work implements. Further compounding the problem is that the hi-rail vehicle's flanged guide wheel are bearinged to fixed spindles, rather than being a solid wheel-axle-wheel assembly. Those bearings provide a very poor electrical path.

For these original reasons, such vehicles were deliberately insulated, to prevent anyone from relying upon such an unreliable thing in a safety critical situation. Safety thus was always provided by more sure methods. Of course, insulating the wheels had the added benefit of not continuously tripping grade crossing signals while work was going on nearby.

More recently, electrical brushes / scrubbers have been applied to the flanged wheels, but they still do not reliable trip block signals. However, they are useful in triggering modern grade crossing predictors - or not, at the discretion of the operator. GCPs are more reliable in this application because of the natural electrical properties of the predictor circuit, which is much more sensitive to the sudden electrical changes and erratic shunting which the hi-rail truck presents, than is an ordinary block signal track circuit, or even the old fashioned grade crossing circuits.

OPB