My trix track is a very simple circle. Today, the model train suddenly couldn't pass through one of the straight tracks. I started to look for the cause and tested it with a multimeter. There was no problem with the power. I changed to another train and tested it, but the problem still existed, which meant that the problem was not with the train. Then I wiped the track with an alcohol sponge, but it still couldn't pass after the test. I don't know if there are other ways to find the cause?

Possible reverse loop short?

To paraphrase what you wrote: You have at least two locomotives that run fine on most of your single circle layout but stop one one specific straight piece of track. Measuring the voltage on this dubious track shows no voltage loss vs. the other parts of the circle.
That's a strange error, and that's little information.

Some questions:

- Do the locos stop on exactly the same spot?
- Is it guaranteed they don't create a short circuit?
- Do they stop in forward as well as in reverse direction?
- Do they stop if turned around?
- Do they run if power is applied directly to the track piece?
- Did you exchange / replace / reverse the dubious track pieces?
- Did you check the continuity / resistance of the track connectors?
- Did you measure voltage with some load applied consuming, say, 0.5 - 1 Ampere?
- Did you measure at various voltage settings?

Wouldn't that short circuit the entire layout?