Nearly all of the streetcars in SF in the 60s and 70s had flat wheels.

Nearly all of the Cal Cars on the Capitol Corridor have at least one set of flat wheels.

Observations from personal experience.

Some PCC operators on the Muni kind of polished off the flat spots by doing full spin starts adding sand only after the spinup was complete. Impressive accelerations at times. Well before traction control systems were invented let alone became common.

No longer legal, but I was on one late Sac RT train where the driver would maintain track speed until nearly entering the station, then lock everything (including the track brake) up in emergency and slide to a halt exactly at the handicapped platform. Did it several times on that trip. Made up several minutes of the deficit over a few stations. After the first one, people getting off held on very tightly. No longer legal (probably wasn't legal then) because PUC insists that the train enter the station at no more than 20 mph, with local rules encouraging 15; people wandering around the platforms have been hit in the past. Compliance with station speed limits has extended the running time on the Gold Line by at least 15 minutes from what it was when opened.