Caltrain has a membership in the University of California at Berkeley's Rapid Earthquake Data Integration system. The computer is located in the Caltrain Central Control Facility in Menlo Park.
Since 1994, Caltrain has used the REDI system to identify the magnitude and location of earthquakes and their possible impact on Caltrain's tracks and structures. Without REDI, Caltrain maintenance of way staff would have to physically inspect all 50-route miles between San Francisco and San Jose.
Magnitude 4.5
(within a 50-mile radius) alarm will sound. No action needed.
Magnitude 5.0 - 5.7
If within a 50-mile radius, operate at restricted speed. (Restricted speed: movement must be made at a speed that allows stopping within half the range of vision short of an item on the tracks or bad tracks. Train cannot exceed 20 mph.)
Magnitude 5.8 - 6.2
If within a 50-mile radius, stop until track can be inspected. If within a 100-mile radius, operate at restricted speed.
Miscellaneous
Tracks south of San Jose Tamien are inspected by the Union Pacific Railroad.
Tracks are inspected using a hi-rail vehicle. It would probably take a couple of hours to inspect the tracks (this also depends on how much damage is out there).
Inspection crews look for anything unusual, i.e. anything that wasn't there the last time they looked. The crews also check the condition of the tracks, the bridges, the tunnels and look around to see if there are any potential hazards, i.e trees or electrical wires ready to fall.
Direction of inspection (from one end to the other or both ends to the middle) depends where the hi-rail vehicles are, magnitude of the earthquake and where the trains are.
Resources
Visit the Earthquake Country Alliance website to learn the seven steps for earthquake safety:
https://www.earthquakecountry.org/sevensteps/