The biggest problem is indeed cost. Installing platform doors on the platforms throughout the system has been estimated in the past at US$1 million per station

The biggest problem is indeed cost. Installing platform doors on the platforms throughout the system has been estimated in the past at US$1 million per station (US$400–500 million) in 2012 [1] and US$1 billion in 2013 [2]. But those estimates seem incredibly low now given more recent per-station costs elsewhere (CA$10 million, €2.6 million) [3], and the fact that NYC subway construction is higher than most other major cities [4]. As for depth, most NYC subway stations are directly below the street. I’d be surprised if NYC builds platform doors before Communications-Based Train Control (CBTC). [1] https://www.wnyc.org/story/283792-nyc-mta-tepidly-explores-platform-barriers-after-subway-track-deaths-again/ https://www.wnyc.org/story/283792-nyc-mta-tepidly-explores-p... [2] https://www.nytimes.com/2013/01/29/nyregion/mta-says-subway-platform-gates-could-cost-1-billion.html https://www.nytimes.com/2013/01/29/nyregion/mta-says-subway-... [3] https://www.thetransportpolitic.com/2017/09/26/the-case-of-the-missing-platform-doors/ https://www.thetransportpolitic.com/2017/09/26/the-case-of-t... [4] https://www.nytimes.com/2017/12/28/nyregion/new-york-subway-construction-costs.html https://www.nytimes.com/2017/12/28/nyregion/new-york-subway-...