Even SF Muni which is heavily used has only a 22% farebox recovery ratio.
It's only in above ground systems where you have to have people watch against fare-beaters. But even then the tickets are mostly electronic and self-serve.
And in many areas of the USA, I doubt removing fares will push demand past capacity -- it's not the $100 - $200 month in fares that keeps most people in a car that may cost them $700/month or more. [1]
[1] https://www.nerdwallet.com/blog/loans/total-cost-owning-car/
Especially the WWII bomb thing is annoying, because they tend to find one every few weeks when there's construction near the tracks.
However, if the transit system is already at capacity at peak time, then it would need to be restricted to off peak (in lieu of building more services), as the ticketing is acting as demand management rather than revenue collection.
What is interesting is that most public transit obeys the same congestion rules as roads - if you take a congested road system and add more roads, you often just end up with more cars on the roads at the same times congesting the new roads again because of all the offset load that uses transit at off hours due to congestion. For people on the roads, they are effectively "free" to use once you have the car, but people still actively decide to drive at off hours to avoid congestion. The same happens with any transit system regardless of cost - unsurprisingly, people don't want to be stuck in traffic.
It’s not exactly the same idea, just a similarity from where I’m living.
It accomplishes a ton:
* It gets people who cannot afford regular fares to use these services when appropriate, rather than what might be less desirable (having a friend drive them around if they are too poor, etc).
* It gets people using services they might not use otherwise. A lot of commuters would take the train once or twice just because its free. If you can get people started on using public transit, they might just keep using it.
* You can also give away free rides during low density hours, such as at night. This can help offset daytime load by getting people to do their time-flexible commutes at off hours.
Now there are a lot of tweaks on the model, like making free rides only eligible for non-peak hours. But the whole point of public transit is to increase the efficiency of people and business in the economy where it is built - it is never meant to turn a profit, and if it is profitable it is probably failing on that first premise by costing so much to use. Making it completely free all the time makes it rife for abuse unless you institute a system where riders who ride the most get the least priority. But honestly if anyone were willing to implement such a system making the transit free and just using the tax revenue of booming business to pay for the transit is a much more sensible model than trying to keep your transit system revenue neutral or even cash positive (cough, Amtrak).