Good luck finding that type of insurance! I have 2 motor cars and belong to NARCOA. We must carry insurance through NARCOA to operate on any of their excursions. This insurance only covers damage to the hosting railroad's property and only when on a NARCOA sanctioned excursion. This does NOT cover the operator or passengers in the car. We, the operator and passengers of that motor car, have to sign a waiver for every excursion releasing NARCOA and the hosting railroad of any liability that may happen. Basically your on your own!
We can also obtain an optional insurance policy to cover any damage to our cars if something does happen.
If we use our motor cars on a non sanctioned NARCOA event, the host railroad will carry an insurance policy for the duration of a specific event.
If it is at a museum, their insurance may cover you if you are a member BUT I not sure on this! That would be something you may have to check into.