For heavy rail, LA's Red/Purple lines actually have pretty good ridership per mile, higher than DC, Chicago, and SF BART.
For light rail, Houston's METRO Rail has slightly higher ridership per mile than LA's Blue/Green/Gold/Expo lines. Dallas' DART is way down on the list.
In terms of commuter rail, Dallas' Trinity Railway Express has higher ridership/mile than LA's Metrolink.
Source:
https://www.citylab.com/transportati...ervice/576739/