In fact, MTA have been investing in vacuum cleaning systems for the tracks with the aim of reducing fires that lead to delays. You're being disingenuous. Here's an article:

https://www.nytimes.com/2017/07/24/nyregion/with-new-vacuums...

So it's frankly absurd of the commenter to say that just 4-5 people could manage this task. One manager mentioned in the article leads a team of 300(!) cleaners.

That's the subway tracks. They are forced to clean debris off the subway tracks, otherwise the trains can't run.

But where the people are, the stations, they don't keep them clean. That's what the commenter was refering to.

They do have those teams. I've seen them work; they've got power washers, and they blast the stations. They cleaned my previous-local station once every few months, at least.

The stations don't stay clean because they move millions of people per day, not all of whom are clean. Today I saw a kid throwing up on her way into the station; she and her mother were somehow polite enough to pause above a drain to do so. Combine that with people tracking upstairs dirt in, and littering, intentional and accidental (I dropped a candy wrapper while getting my kid home; I'm not stopping to pick it up mid-tantrum), and you've got a pretty messy situation.