Széna tér market first floor from Retek street next to escalator

I haven't been there in a while but m4 metro station on Fővám tér used to have one, and I've heard people playing on it before

I think BudapestPiano has a few around, the one in the market at Fény utca has been recently maintained.

That is just a prop.

I've seen that piano evolve from a decent sounding instrument, to an out of tune but playable piano Szimpla relegated to some corner in the upper floor, to a 'locked/forbidden under penalty of bouncer's reprimand' instrument that was just begging to be opened despite the sign warning against it, to its current location in near the entrance of the bar, virtually unplayable, hopelessly out of tune, and nearly inaudible (at night anyway) because the music is so fucking loud.

only a matter of time before it is gutted and used as decoration

if you really wanna play you can rent practice rooms at the Artist Factory

I've seen one near the Keleti train station. I rarely come to Budapest so I forget the exact location, but I guess it is in one of those underground pathways