That's interesting, why would you put it along the track and not on the train itself? In the 60's it made sense but today wouldn't it be cheap to put digital temperature sensors on the trains like cars have?

Probably cost. There are millions of rail cars, both here and in Canada (not sure if the railroads run trains in Mexico) that would need to be taken out of service and fitted with them. Plus, easier to test to see if the detectors are working - just run a work train past them with a resistance heater hanging off it to simulate a hot bearing.

And -- the sensors would have to communicate with the crew if there's an alarm, and they could be a half mile away (long trains are long) inside a tunnel. There aren't any data connections between cars (and wouldn't be reliable if there were), so the on-car detector would have to have a powerful radio. Added cost & complexity.

Are journal bearings still common on American rolling stock?

Probably not, but modern bearings can still fail, and the result would be just as catastrophic.