Caltrain spokeswoman Christine Dunn said southbound train No. 102 broke down at the Belmont station at 5:48 a.m.
At 7:15 a.m., southbound train No. 206 had mechanical problems at the San Carlos station and was taken out of service.
Then, at about 7:50 a.m., southbound train No. 314 broke down at the San Mateo station, Dunn said.
Passengers on the disabled trains were transferred to other trains, but the problems have resulted in lengthy delays to both north- and southbound service because trains had to single-track through some areas.
Delays are expected to continue throughout the morning commute.