The Train is the name given to an unidentified sound recorded on March 5, 1997 on the Equatorial Pacific Ocean autonomous hydrophone array. The sound rises to a quasi-steady frequency. The origin of the sound is unknown.[1]
Some of the other significant unexplained sounds named by NOAA along with train are Bloop, Julia, Slow Down, Whistle and Upsweep.[2][3][4]