For those of you with high-rail pickup trucks on heritage tourist lines: what kind of insurance do you have and what does it cost?

Commercial insurance $500k liability
Wasn't worth getting full coverage as most hi rails in the used world are sub 10k vehicles. I did have the insurance company see photos of the vehicle first. And the coverage does not cover rail mounted incidents.
Go to a local insurance broker and talk to them and be up front about everything

Btw about $100/month

You might be able to cover it under your Inland Marine policy.

Talk to your insurance person. Policy types, coverages and rates are all over the place.