Officials tell the Greenwich Time that the dog — nicknamed Metro — had a microchip implanted in his shoulder in case he got lost. The data from the chip showed that Metro was once a puppy in Maryland who was named "Brownie."
Animal Control Officer Vincent Pennatto discovered the chip with an electronic scanner, the Time reported.
"Metro's" first owners adopted him in 1997, but say they gave the dog to another family who had a handicapped child. Nobody's sure how he made it to Connecticut, or how he got onto the train.
An animal control officer responded to several Christmas Eve sightings of a dog wandering Greenwich train stations, but every time he arrived, the dog was gone. A commuter caught up with him after he hopped off the train in Harlem.
Dozens of people have called offering to adopt Metro, but officials are still trying to find his owner.
The red-haired spaniel is living in a Greenwich kennel while the search continues.
But Pennatto said the dog's owner won't automatically get "Metro" back.
"Whether they get the dog back will depend on whether they seem like good owners who lost their dog or somebody who didn't really care," he told the Greenwich newspaper.