A teenager has been hit and killed by a train in a tunnel at Lewisham station, where police believe he was spraying graffiti with friends.

Police were called to the station about 9.30pm yesterday, after reports a train driver had hit something.

Officers found the body of an 18-year-old man in the tunnel of the West Street overpass.

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"It is believed the man may have been graffitiing inside the tunnel with friends when he was hit by the train," NSW Police said in a statement.

Four men went to Glebe police station soon after to report the death, and they were being questioned by detectives.

The train driver and guard were breath-tested and the results were negative.

Police said the teenager had yet to be formally identified and his family had not been notified.

Several of the boy's friends paid tribute to him on Facebook.

"RIP ... will never forget ya man," one friend wrote.

Another wrote: "rip brother love you man."