The National Institute for Occupational Safety and Health wrote a paper evaluating the accuracy of iOS app decibel meters. They found that the best ones match professional decibel meters within about 2 decibels. After doing all that evaluating, NIOSH made their own app that’s also within 2 decibels of professional decibel meters and will tell you how close you are to damaging your hearing.
There’s a sliding scale where you can hear a given volume for a given amount of time without damage. The threshold for an 8-hour shift is 85 decibels of constant noise. The NIOSH app will tell you what fraction of the daily dose you’ve gotten to while you run the app and will project what dose you’ll get to over 8 hours. I ran the app on BART and found that when I wasn’t in a tunnel it was well below the 85 decibel limit. I ran it a few more times back and forth through the tunnel and found that it’s loud enough to damage your ears if you do nothing but ride through tunnels for 8 hours, but even for a BART driver there’s probably no risk of damage.