Last I checked governments have the ability to tax and regulate commerce for the common good, so I don't know why you're implying that its unconstitutional. That's just like saying governments building highways, rail lines, ports, and airports are wealth distribution in the other direction, as the rich who own the transportation industries make more money off of them. It shouldn't matter. Time is money, and efficient transportation saves a lot of it; whether its public transport or your own vehicle on a street the government built I don't care.
The Congress shall have Power To lay and collect Taxes, Duties, Imposts and Excises, to pay the Debts and provide for the common Defence and general Welfare of the United States; but all Duties, Imposts and Excises shall be uniform throughout the United States;
Common Good isn't in there. the general Welfare of the United States is referring to the entity, not the population. Common applies to defense only. Wealth redistribution isn't in there, and was explicitly warned against by the people that wrote this, so don't bother regurgitating the half baked bullshit argument liberals always throw up at the first sign of someone being capable of basic reading comprehension.
To regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes;
Here we have the idiocy used to justify the interstate highway system. Regulate does not mean what they claim it does, banning shit left and right as they micromanage the economy into the ground. Historical documents must be defined by the period, not current shenanigans to make things say something other than what they did. Regulate means to make regular. Congress has the power to make certain that interstate commerce remains open and fair. They're supposed to stomp on states that impose tariffs, and other forms of idiocy to gain a competitive edge over their neighbors, such as requiring in state drivers. It's a crap argument, this doesn't even justify the interstate highway system. To the effect of this argument though, it's bloody wonderful. Your local putz city bus system has fuck all to do with interstate commerce, thus it's not covered in any way and cannot be promoted or restricted in any form by the federal government
I said the interstate highways aren't really justified by the commerce clause, but they are justified. Interstate highways are necessary for the transport of mail. Airports are necessary for the transport of mail. At the time the rail system was being built, rail was used for mail. Problem solved. Our government isn't bright enough to figure that out though, so they came up with a convoluted explanation for the above instead of using this:
To establish Post Offices and post Roads;
That's not to say no one can legally run public transportation in the USA. There is no explicit restriction on expenditures for public transportation written into our set of rights. As such, municipalities and state governments are all quite free to play with their own localities barring restrictions in their state constitutions, or local ordinances. It just isn't constitutional for it to be federally funded.