Subway at Union Station; photo from Metro Library via Flickr

How do they do that? is a new series for The Source that explores the technology that helps keep Metro running and passengers and other commuters moving. Some of it applies directly to the trains, buses and freeways and some of it runs in the background — invisible to nearly everyone but essential to mobility in our region.

Following an earthquake, how does Metro check the rail lines to make sure they’re safe?

In Los Angeles we live in a world of nearly constant earthquakes. Most of them are too small to notice but seismic detection equipment along all of Metro’s rail lines is constantly tracking temblors and reporting movement to staff in the rail control center, which monitors train activity throughout the Metro system.

When a quake is thought to be strong enough to cause damage, rail control center staff will radio the train operators and tell them what to do. Orders can vary from line to line, depending upon where the quake is strongest. If a weak quake is centered in the San Fernando Valley, for example, trains in Long Beach may not be affected.

If the quake is deemed potentially damaging, operators may be told to stop where they are and begin sweeping the track, which means that they proceed at about 15 mph to the next station or to the point where the train ahead of them stopped and began its sweep. (In that way, every inch of the track can be examined.) While the operators are proceeding they carefully watch the track looking for damage. Everyone reports back to rail control, which determines if the line or lines can reopen. Decisions are based on the common sense of humans, rather than seismic machines.

Should a significant event occur, the entire rail system would be shut down and not reopen until all lines have been thoroughly checked and determined to be safe. The term “significant” does not refer to Richter scale strength but to a variety of factors including strength and location of the quake and the judgment of rail control staff.

Should operators feel an earthquake (not that obvious in a moving train), they must immediately stop where they are and then proceed slowly to the next station. Or they may be given specific instructions from the rail control center, which generally will tell them to begin sweeping.

So the system is watched over by both machines and humans but the most important element — as in many things — is common sense and caution.