Here is an editorial by Mitchell Moss in the NY Times that describes an unappreciated piece in the perilous structure of transit finance. What happened is that many transit agencies shifted ownership of their vehicles and equipment to banks, and then leased back the equipment. The transit agencies got money up front and the banks got depreciable assets and favorable tax deductions.
These agreements are no longer allowed, and the existing ones are falling apart because the banks are claiming the transit agencies are in default. There is a lot of money at stake and this could be crippling to transit in many cities. The author of the piece suggests that there are some potential legislative fixes to prevent the existing agreements from coming due. Hopefully this is solvable because transit agencies have enough problems as it is.