The answer is simple, but the politics is difficult.
It is pretty obvious that real estate value is greatly affected by closeness to mass transportation, especially that which runs on rails.
Not coincidentally, that is where the most valuable real estate is.
So, I propose additional funding for the CTA come from the real estate tax in areas served by the system.
Those who receive the most value from mass transit would pay the most; those receiving the least value, the least.
Naturally, the best tax is a tax someone else pays.
But a good tax is one that can be logically linked to what it is financing.
A property tax can achieve that linkage to the Chicago Transit Authority; a sales tax cannot.
And, there is one other advantage. The property tax on at least residential property in Chicago is very, very low compared to elsewhere in Illinois.