In most cases, they do so to inspect and clean the metro. They have teams to identify and clear out major trash and 'fluids'. Also, to note anything suspicious. I accidently boarded one when it went out of service and that's what they told me was why they did it.

It isn't long enough for them to do any kind of inspection or cleaning. It's really only 30 seconds to a minute.

Maybe the driver inspects each car via camera from the driver's seat and then puts it back in service? Then if the driver saw something, they could clean it before putting it back in service.

They definitely do inspections at Shady Grove. There's a whole host of metro workers that walk through the cars before it goes back into service.

Maybe you caught them when they were running ahead of schedule and had to delay for a minute

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They are human and have to pee. Probably in a bottle

If it's the last train of the night, the operator is checking for anyone who fell asleep. You do not want to wake up while they take the train to where ever it goes at closing.

That makes sense, it is about long enough to walk the length of the train. I’m pretty sure they used to do it while leaving the train in service though.

Taking a guess here. Maybe in the past, when metro had more staff, the turnaround would also be the point where the train operator changes, giving the incoming one a break. With two operators, you could effectively minimize the time the train sits out of service as control changes from one end to the other.

But now with the staffing shortage, you're more likely to see the same operator operate the same train on the way back, meaning there's a more noticeable amount of time the train is "out of service" while the operator traverses the length of the train.

Inspections. Usually like 4-5 workers go in and just sweep up the noticeable stuff and make sure there’s no wild shit