I take my folding bike onto the subway a few times a week.
For the last 5 years, it has been a SunRace folding bike I bought from Nashbar for $250.
Aluminum frame, 8 speed, mechanical disc brakes, nothing fancy, just gets me from point A to B, very dependable and easy to carry.
I most fold it only when I need to put it in the car trunk/hatchback.
I cut the handlebar to be narrow, so I can easily fit between car mirrors in NYC traffic, also take up less space when folded.
Recently, I bought another 20" folding bike from FB Marketplace for $150:
No particular name brand, aluminum frame, 8 speed, mechanical disc brakes, easier to fold,
feels lighter than my previous SunRace that's 5 years old, which now I keep as a spare bike.
Because of the low cost of these folding bikes, I rarely even lock them when I run errands, just fold it up,
take off the seatpost/seat and carry the seat/seatpost with me into the store, no one seems to bother to steal these cheap folding bikes.