According to the Center of Disease Control and Prevention, 37% of New Yorkers between the ages of 18-64 years old are handicapped.
Wheelchair accessibility in New York subway stations are not completely scarce. There are 38 subway stations in Manhattan that are accessible to handicapped people and most of them are in major locations of high-ridership.
Stations outside of Midtown, such as the 168th Street and 181st Street locations in Washington Heights, are only accessible by stairway.
Parks, train stations and playgrounds are some of the most common places outside of subway stations that have publicly handicap accessible bathrooms.
http://cdb.io/1CTmkfv
http://www.cdc.gov/ncbddd/documents/Disability%20tip%20sheet%20_PHPa_1.pdf