Why would that be needed? I visited NYC the first time as a high school student with not so perfect English as a foreign language skills in 1980 when the city was in a pretty rotten state. The last time in 2009 when the subway was at least free of graffiti and generally looked maintained better. In younger years I found huge subway stations and their connecting walkways fascinating so might have planned some trips with extra complicated changes :) I never had the feeling of getting lost although I definitely lost all sense of orientation. It was easy enough to just "blindly" follow the signs. Not sure why an app would be needed, I assume the signs have not disappeared.
(In 1980 some connections were still sign-posted as "connection to IMT", ... BRT, or ... IND lines although those had ceased to exist 40 years earlier and where not marked on the official subway map. But even that you could just guess from the "breaks" in the line numbering.)
It’s unusual to see someone try to make the case for the MTA not being complex, “I got lost in the subway” is like a lowest common denominator NYC experience.
If the only previous subway experience you have is Glasgow (a single circular route) then yeah, New York is potentially intimidating, but I don't expect a Londoner or Parisian would see it as disproportionately complicated. More stations, but not orders of magnitude more. Tokyo too, and New York only has (these days) a single integrated system so no weird ticketing or line change rules.
Not if all the trains are running normally. The problem is that they often (usually?) aren't.
The complexity isn't the map. It's the fact that trains shut down, swap tracks, switch lines, skip stations, end early, etc.
I've never seen any other city where you have to stay on top of the constant changes. During weekdays it's not so bad, but nights and weekends yikes.
Earlier the first thing in a new city was getting a paper map. No matter whether free or for purchase. Studying the map prepares you for following signs and announcements. Unfortunately in many cities you cannot get paper maps anymore. A phone screen is 1000 times worse ergonomics to understand a network. Often I print maps before going to a new place.
I recently travelled 2 weeks by train in foreign countries. I did not turn on GPS a single time. It's just a cause and symptom of helplessness if people need that. Yes, I used my phone to make hotel reservations. 20 years ago I did the same using tourist offices, phone books and payphones. But the difference is really marginal. Travelling was not a problem without smart phones.
Which there frequently (usually?) aren't. Because so many disruptions are unplanned.
Also it's a big waste of time to walk 10 min to the subway station and pay the fare only to discover the train you need isn't running and you needed to go to a different station on a different line.
These days, it's really easy with Google Maps. Just tell it where you want to go and follow the directions. It tells you the station entrance, which platform to stand on, which car to ride in, when the train is coming, if there's a delay, and which exit to use.
Navigating it for the first time is daunting.
Very different from most single-line stations.
Certainly there is a mechanism where if a phone can get a GNSS/GPS fix, it can then listen for other signals like Wifi and cell towers, and correlate the two: so if GPS isn't available other stuff can heard and used for location.
But if there's no GPS, but there are other signals, how can you map the signals to an actual location?
You'd have to have someone (Apple? Google?) go in with survey equipment and map the non-GNSS (beacon) signals to locations 'manually' and put them in a database.
And concidering cell signal is likely covered down there and the locations of cell towers are almost certainly mapped they could use some sort of triangulation to generate a pretty reasonable map even without having ever been there.
As I said, I doubt they would put the money into it... Would be a really fun project to build out if you had access to the data though... Maybe don't want to do it because it would make it more common knowledge about how easy it is to track device location even without GPS...
I don't think you would need physical beacons, cell towers or fixed wifi APs[1] will likely be good enough and using some statistics would make removing cases where an AP moved/get renamed / goes down temporarily.
1. Access Points
I'm saying it's likely not needed.