Gas taxes and toll revenue account for about 1/3 of road infrastructure spend these days. This is significantly less than years past as the fixed gas tax hasn't kept up with inflation or adjusted upwards enough to match efficiency gains.
My state gathers money for a highway fund from gas taxes, vehicle registrations, vehicle sales taxes, etc. They have a program called "Municipal State Aid Street" where they reallocate some highway funds for city and rural streets. There's even a map. [1] I'm quite sure that everywhere has some similar program.
There's this public transit chauvinist myth that roads are, "100% subsidized" as you say, and it's wrong. Stop making things up or just accepting vague collective wisdom. Roads are paid for in significant portions by road users. Not 100% because that would be an undue burden on the poor.
Trains, despite being crazy expensive and heavily subsidized, are more expensive to use than owning a car for a lot of people or they are enormous money sinks that are paid by non-users. They can be incredibly useful and a worthwhile investment, but I've definitely run the numbers in the past and found the train to be more expensive and much more unpleasant than driving (for me it was a Caltrain+BART commute).
You actually can, but the hassle is expensive. You have to coordinate it with the railroad and the controller and you have to provide a locomotive and an engineer and on and on and on, but you can do it.
For heavy rail, yes. For light rail, that is more likely to have been built with local funds, is this actually a thing --- I'm sure if you work with the film commission, but I would be surprised if any light rail systems have a plan for private vehicles.
All light rails either operate like heavy rail (controlled from a central control area, receive clearances and obey rail signals) or they are just busses on tracks and operate by the rules of the road. Some are both.
The second are easy, and you just need permission in general. The first you need communication with the central office.
This is all done for maintenance vehicles for the tracks, which are often contractors.
At the time of making the video this line had been closed for over 20 years, which is why it's so overgrown and why nobody bothered to disturb the riders.
City streets and roads are paid for through property taxes, so they are subsidized 100%.