That's consistent with my recollection riding BART. Shutting down one of the two tracks for unexpected daytime maintenance ("single-tracking") is a common source of delays, since in single-track mode the system can't run nearly as many trains per hour due to the need to pass at specific points. But it can still run some trains, probably enough for a night service.
It's definitely true. BART has one track in each direction. Systems that run 24 hours have dual-tracks in each direction or segmentation that allows them to individually power-down segments of track, though that isn't really 24/7 operation anymore.
You can research this but I'm not going to do that for you. I'm comfortable that I'm correct, if you're not, well, meh. It's not personal, I'm sure you can understand.
And to your original point -- the engineer driving the train is a fraction of the cost of keeping the system running. Even if BART was dual-tracked, the salary of the engineer doesn't hardly move the needle on their costs, which include: Salaries of station agents and security, additional maintenance and wear-and-tear, janitorial (including dealing with higher janitorial costs between 2AM and 6AM than other 4-hour periods due to the nature of overnight passenger traffic), baseline power costs to keep the power rail hot at 1000+ volts over 100+ miles of track, etc.
Oh, yes, it has one track in each direction. I thought you were arguing it was actually single-tracked in portions, the way some commuter-rail lines are, and which was the original proposal for the SFO extension. The Copenhagen Metro is also one track in each direction, and runs 24/7. At night it will sometimes (if maintenance is needed) run in single-track mode, with one of the two tracks powered down in a segment, the same way BART operates during emergency daytime single-tracking. Perhaps that's not "really" 24/7 operation, but it does produce the effect of uninterrupted 24/7 service from the passenger's perspective, since it's able to maintain its scheduled nighttime headway of 20 minutes even when single-tracked through a segment, barring exceptional circumstances.
That's consistent with my recollection riding BART. Shutting down one of the two tracks for unexpected daytime maintenance ("single-tracking") is a common source of delays, since in single-track mode the system can't run nearly as many trains per hour due to the need to pass at specific points. But it can still run some trains, probably enough for a night service.