You're presuming that most of the passengers are in fact paying. The last several times I've ridden on the Mitre line to Tigre, there's not been anybody bothering to check at either end.
My wife tells me no one buys tickets on the trains (well, very small percentage) and no one checks.
We went across town recently (in fact, the only time I've ever ridden the train here) and we bought tickets. We left from Retiro. There was almost no one buying tickets but there were a lot of people heading into the terminal and going straight for the trains (it was outbound rush hour on a weekday). Once on the train I was talking to her about this and people all around us were hearing what I was saying and sniggering at it.
The train was crowded - many more people standing than sitting and I felt squished (that's why I bought a car - public transportation is fine, but I like my personal space - and my personal valuables). No one was going to check for tickets - impossible.
I doubt very much the trains would run any better under national management. It takes a lot of money to maintain trains and pay the operating and personnel expenses and I think the government has other problems on its mind right now.
There are so many problems wrapped up in why the trains are poorly maintained and dangerous, all having to do with lack of money getting to where it needs to be. Corruption is the key, private or public, and no one really being held responsible (up to and including allowing the union to overwhelm any safety procedures and requirements) for what matters.
And it will stay this way as long as the people continue to tolerate it. That includes the people who think that public transportation should be "free", which seems to be the majority of the people here. A lack of people paying taxes (and I understand why - with such a corrupt government who really thinks their money is well-spent? but that doesn't excuse them not paying to get on a train and using a service directly when money is supposedly required), a lack of people paying when they get on the train. No enforcement. Powerful unions with high-paid employees. No money left for maintenance and operations. And so on and so forth.
The people don't want to pay anything and they aren't forced to.
If a private corporation did not receive subsidies (of course, it isn't really private if it receives subsidies, is it? Isn't it more of a client, particularly because the government also controls the price?), it would make sure it collected money for its operations. If it was held accountable for accidents, it would make sure that its drivers were awake and paying attention, even if a camera with a watcher was needed. It would spend at least the minimum money necessary for important maintenance. And so on.
Oh, but wait! I forgot. The government wants a low cost transportation system and therefore controls the price. They try to offset the difference with subsidies but don't have the money. The strong unions set the price on wages and a lot of policy. Yeah, can't work.
Don't get me wrong, I know low-cost public transportation run (or controlled) by governments can work - but it requires a relatively non-corrupt society and government to happen.
Of course here, even privately run transportation isn't really private. That's why the prices are so low (and quality so poor) while the prices on just about every other consumer item is so high with at least some relatively quality items as well.