Something like this maybe?
http://philly.curbed.com/archives/2014/10/23/korean-company-...
I'm not sure how many ads you have to sell to recoup the cost, but it's probably a lot.
> on the order of >$1M per platform
That's insanity! Over $1M for something every amusement park has for every ride? Lemme guess, these one have to be engineered to the point of 'impossible to fail' and installed by the employees of municipal cronies at 3x fair wages?
Talk about perfect being the enemy of good...
It needs multiple sensors that can handle being exposed to a lot of wear and tear. It needs to operate for years on end with, ideally, very little maintenance. It has to endure being buffeted by the forces of the train entering and leaving the station which exerts significant strain on any hinges or track mechanisms.
So you need a pair of these for each set of doors on the train, and if each car has three doors, and you have eight cars, that's 48 doors.
That leaves you with only $20,800 per door. You still need to build the walls themselves and you need to have this installed by the right technicians.
Do the math, don't bitch just because it sounds expensive.
The door must detect a 1-2cm obstruction (e.g. fingers, child's wrist) and be sufficiently weak that a child can pull their hand/wrist free. The door must not prevent a train from departing if only a hem of a skirt/trousers/coat is trapped. They must be strong enough to prevent anyone falling from the train while it is in motion, and must not be able to be forced open between stations.
Otherwise, approximately one person per year will die.
How much would you charge, to make doors to this specification?
And, some time in the next five years, an accident will happen, somewhere in the world. Your next door will need to prevent that situation, too.
[This is the situation in "wealthier" Europe, where it's pretty difficult to die on a train. Other countries may vary.]
If the padding is too squashy, a trapped hand won't be detected and the person could be dragged under the train. Too sensitive, and someone leaning a suitcase on the door prevents the train from leaving.
But really, you don't even need a door - a simple gate like on a ride at Disneyland, or heck even the gates at the station entrance would do nicely.
Furthermore, platform doors require you to have trains that have the doors in the same positions on the car body, which much of the newer subway fleet (R142,R188 & R143,R160,R179) has but the legacy fleet does not. This issue get compounded even more where multiple lines intersect (which is often due to the NYC subway's unique connectedness) requiring that certain stations would need to support 3 or 4 different car classes.