How would you actually find everyone in a train car who traveled with a complete stranger? That’s not possible. Any more than tracking everyone who was in a grocery store when one shopper caught the virus.
I believe the criteria for quarantining is coming in close contact of a positive person within 6 feet for 15 minutes. In my town, you are better off taking the bus than ride sharing. I never see more than a couple of individuals on the entire bus during rush hour..