This time of year, Bryant Park is flooded with holiday shoppers strolling amid the vending booths and skaters gliding on the ice rink.

Few, if any, might have any inkling that just below them lies a world-class repository of research materials in a concrete-encased, climate-controlled book vault. And a little red railroad.

The recently opened book bunker with 55,600 square feet of space extends west from the main branch of the New York Public Library on Fifth Avenue. That little red railroad winds from this underground bunker up through the cavernous building.

The train is not some decorative holiday touch, like the wreaths that get placed on the two marble lions that guard the library’s main entrance, but a sophisticated book retrieval system that chugs along behind the scenes, seen by only a small number of employees and sharp-eyed users.