There comes a moment for riders of the PATH train, a shared, unspoken experience when the air is pierced by a distinct odor.

Sometimes it hits while descending from the street to the station. Sometimes it strikes inside the train. Then just when the nose adjusts, another distinctive smell emerges as the train goes beneath the Hudson River.

For decades, PATH riders have wondered about the smells of their trains. Brian J. Cudahy mentioned the smells in his 1975 book, “Rails Under the Mighty Hudson.” He speculated that PATH stations smelled different from New York City subway stations because they tend to be deeper underground, and posited that a moldy odor could be caused by Hudson River water seepage into the tunnels. The only thing he could confirm was that it was distinctive.

“If you had me blindfolded in 1950 and you walked me into Hudson Terminal, I would have known where I was,” Mr. Cudahy said in an interview. That is because while the subways “always had the smell of brake shoes,” PATH trains smelled of “a touch of dampness.”