Sources:
* https://www.nytimes.com/2017/12/28/nyregion/new-york-subway-...
* https://jalopnik.com/heres-the-most-damning-report-yet-on-wh...
* https://www.washingtontimes.com/news/2018/may/14/new-yorks-s...
> the implication here is that the NYC subway's sole financial problem is corruption. Is that actually true?
So, that isn't the assertion. The argument that taxes are raised for a project, then being diverted to general funds is a separate issue from the one of corruption, and speak more directly to OP's argument that "taxes don't go down", to which the rebuttal from GP was "Well, taxes shouldn't go down, because maintenance is a thing". If the taxes don't go down, and also aren't being used for the purpose they were raised, you have sort of a shell game.
That said, on the matter of corruption, here are a few quotes from the first source I linked:
> The estimated cost of the Long Island Rail Road project, known as
> “East Side Access,” has ballooned to $12 billion, or nearly $3.5
> billion for each new mile of track — seven times the average elsewhere
> in the world. The recently completed Second Avenue subway on
> Manhattan’s Upper East Side and the 2015 extension of the No. 7 line
> to Hudson Yards also cost far above average, at $2.5 billion and $1.5
> billion per mile, respectively.
> The spending has taken place even as the M.T.A. has cut back on core
> subway maintenance because, as The New York Times has documented,
> generations of politicians have diverted money from the transit
> authority and saddled it with debt.