What happens when a public service becomes free? The people who use the service no longer need to pay for it. This does not, however, mean no one pays for it. The money simply comes out of other people's pockets. People that do not always have to use said public service. Who you may ask?
You. The taxpayer.
Public Transportation cannot be free, because there is no such thing as 'free'. There is passing the cost off to another individual, but there is no free. The drivers need to be paid, gas to be bought, vehicles maintained, and so on. Of course there is the back route, which is advertising. However, if you weigh the profits of advertising on the side of a bus, to the costs of the entire operation, you would still find a shortage, and would still need to reach for more money.
When the government needs to pay for something that they offer free to the public, they will simply raise taxes, and cut budgets, to get the money required. So instead of only those who require the public transportation paying, everyone who pays taxes is paying.
Although public transportation is a good way to preserve the environment, the answer is not to provide it as a free service. There are simple ways to cut back and bring in more profits, therefore allowing the transit to lower the fees required to ride.
I am sure that individual who does not require public transportation, owns their own car, or does not have any need for transportaion, does not want to pay for the price of a ride for several thousand inhabitants of their city who require the public transportation. Perhaps some sort of discount for certain public transportation could be installed for those who use it. However, it is not fair to pass on the responsibility to those who do not use the service.
For those who are using public transportation, t he money they invest is equal to, if not less, than the price for gas. And so, the idea of making it free, is like suggesting to provide gas as a free service. Of course, this is out of the question, but it poses the same idea, and therefore is an excellent example of why free public transit is also out of the question.
And so my final answer is a no, for the sake of all those who do not require the transit, and so do not wish to pay for the next persons 'free' public transportation. I think it is fair to say that it is an optional service, and if you choose to use it, then you are also choosing to pay for it.