At about 7:30 a.m., the man was hit by northbound train No. 215, Caltrain spokeswoman Christine Dunn said.
The incident has caused delays of up to 45 minutes, Dunn said.
North- and southbound trains are single-tracking around the scene of the fatality.
Passengers boarding at Hayward Park and Hillsdale stations are advised to board from the southbound platforms, Dunn said.