Twitter is aflame over afternoon Caltrain delays. One theory? A bomb threat was called in, stopping service. According to rider-reported @Caltrain, "Supposedly the delays are due to investigation on a reported bomb threat. Hopefully, Caltrain will issue statement soon." And @mchow1973 writes, "#Caltrain received a call this afternoon that a northbound train might be carrying an explosive device. That train is now stopped at SSF."

We'll update as soon as we know more. Also, prepare yourselves for a possibly long ride home, train riders.

Update 5:05: There is currently no service between Millbrae and San Francisco. All trains are local. BCN (via CBS 5) reports: "A person called Caltrain about 2:45 p.m. claiming there was a bomb on one of the southbound trains leaving San Francisco, Caltrain spokeswoman Christine Dunn said."

Caltrain riders are recommended to take BART to Milbrae and then transfer to Caltrain.