The victim was alone on a platform before he fell on the tracks for reasons not yet explained just before the train passed by, Bern cantonal police said.
Police said they received a report about the incident at 12.50pm.
The passenger train, which does not stop at Roggwil, was travelling from the city of Bern to Olten.
Although the driver immediately pulled on the emergency brake when he saw the man on the tracks “the collision could not be avoided”, police said.
Emergency services staff who arrived at the scene found the man dead.
Police believe the incident was an accident, the ATS news agency reported.
The case is being probed by the regional prosecutor for Emmental-Oberaargau.