A southbound train headed for San Jose broke down at the California Avenue station in Palo Alto at about 9:20 a.m., Caltrain spokeswoman Christine Dunn said.
As of 10 a.m., the train was about 40 minutes late and was still blocking the southbound tracks, Dunn said. North- and southbound trains were single-tracking around the disabled train.
Passengers on the broken train were getting off at that station and were being told to board another southbound train, which was about 15 minutes behind schedule because of the incident.
Crews were working to remove the disabled train from the tracks.