The problem prompted Caltrain officials to take southbound train No. 228 out of service in San Francisco, Caltrain spokeswoman Christine Dunn said.
Passengers were transferred to another southbound train, No. 230, which will make all local stops to the Diridion station. The train that went out of service was only scheduled to make some of those stops.
A third train will also be delayed by the problem, but Dunn said she hopes no other trains will be impacted.