(AP/Alex Brandon)

Question: Today we’re curious about why trains come to a stop at a station and then lurch forward a few inches or feet. What difference does it make?

Answer: It goes back to the deadly June 22, 2009, crash of a Red Line train into a stopped train between the Fort Totten and Takoma Metro stations.

Metro suspended its automated train system after federal investigators determined a faulty track circuit module led to the moving train not automatically stopping for the stationary train in front of it.

Metro is paying a consultant $1 million to study bringing back automated train operations, but since the crash, trains have been run manually.

The main problem with this, WMATA spokesman Dan Stessel tells us, is that if the operator overshoots the end of the platform, some of the doors in the front car will be past the platform, inside the tunnel — obviously not a great place for people to get on or off.

An operator couldn’t back up without walking through all the cars and running the train the other way from the cab at the other end. And even if they were to try, the system would shut down to prevent the train from going the wrong way.

So it’s a lot better to stop a little short than go too far. But the new eight-car trains are 600 feet long — the same length as the platforms. So the operator has to inch up, or the back cars will be short of the platform.

Even though six-car trains, which run about 450 feet long, could stop before the far end of the platform and still have room in the back to clear the tunnel, operators of the shorter trains are still supposed to go as far as they can on the platform without re-entering the tunnel.

The reason for that, Stessel says, is that operators might run both eight- and six-car trains over the course of a shift. So it’s better to have the trains stop at the same point regardless of the length of the train, so that operators who might mistakenly think they’re driving a shorter train don’t stop their longer trains with the rear cars still in the tunnel

Let us know what frustrates or confounds you about Metro. Reach us at kery.murakami@washpost.com or @theDCrider.