Red Line commuter Jared Sorhaindo wanted to know why the sign at the Shady Grove station often doesn’t say when the next train is coming.

“I find it incredibly frustrating because I don’t know if I have the time to load my card, if I have to run, or if I can afford to walk. I have narrowly missed a train several times because I have no idea when it’s departing. It’s just kind of ridiculous,” he told us in an email.

Metro spokeswoman Sherri Ly said operating the arrival time signs is more complicated at stations at the end of the lines. That’s because they use an algorithm to predict when the next train is coming. At other stations, the system can track the arriving train and predict when it should come. But at the ends of lines, trains could be coming from the rail yard or arriving from the other direction and turning around.

“If the system doesn’t have enough information to make a reliable prediction, it won’t,” Ly said. “The logic here is that it is better to make no prediction than a bad prediction."

She also pointed to the Metro website’s explanation for another peculiarity at the end of lines: a “-” sign displaying for the number of cars on the train. That’s because “the system is unable to determine the next train on the platform getting ready to depart,” Metro’s site explains. “This value will change to its correct numeric value as soon as the system detects the number of cars of the train servicing the trip.”

Got a story about riding Metro or have a question? Send it to kery.murakami@washpost.com or @theDCrider.

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