Police found the bodies on the tracks close to Loughborough Junction station near Brixton, south London, at around 7.30am this morning.
The deaths are being treated as “unexplained” by British Transport Police (BTP), whose officers are investigating the circumstances.
Paramedics from London Ambulance Service pronounced the three victims, one of whom has been identified, dead at the scene.
BTP detectives are trying to establish how the victims, who are understood to all be men in their 20s, “came to be on the tracks”.
One theory is that the victims were graffiti artists who were knocked down by a passing freight train in the early hours of Monday morning.