"Selective door operation is implemented at certain railway stations in the United States. In the New York City Subway, the 71⁄2-car-long platforms at 145th Street (and formerly the 5-car-long loop platforms at South Ferry) are too short to accommodate full-length trains of ten 51.4-foot-long (15.7 m) cars, so only the first five cars of the train opened their doors at these stations."
https://en.wikipedia.org/wiki/Selective_door_operation#International_variations
