That's a bit of an exaggeration. A quick look at the schedule shows they do have multiple trains running in each direction during peak hours:
https://www.capmetro.org/plan/schedmap?route=550
But much of the length is, in fact, single-track, making scheduling hard and meaning if a train is late or breaks down it disrupts the whole system.
And it's honestly pretty silly to see a train with the form factor of light rail but diesel-powered.
Voters did approve a proper light rail system in 2020 but it's gonna take a while to build and has already been scaled back twice, sigh...
What I wrote is accurate. You are referring to a line that goes out of the city to elgin, a tiny town, that is the only part with two tracks. The part that serves the Austin area has only one line and is as I described. Hey what do I know I just lived next to it for 4 years, and drove to work even though I could walk to the train because of how terrible it is and would add 1-2hrs to my commmute.
Err... well... I live in Austin currently.
I'm describing the red line which goes from downtown to Leander. (The one I linked to.) If you're describing some other line then sorry for the confusion, I didn't actually know there was another rail line. There are plans to build a green line to Elgin but AFAIK that's still under construction.
The red line is mostly single-track, but there are several specific segments of dual-track allowing trains to pass each other, which is why they're able to support multiple trains in both directions.
It's still a crappy schedule -- even during rush hour it's still no more than two trains an hour. Supposedly they intend to start running it every 15 minutes once they add some more dual-track segments.
Maybe it was worse when you lived here?