The raised inner lip on the circumference of a steel wheel that prevents it from slipping off the railhead. Flanges guide the wheelset through curves and switches, bearing lateral forces while the wheel tread carries the vertical weight.
Heard on the job
We've got a sharp flange on the trailing axle of that lead motor.
Insider tip
Flanges are not supposed to constantly rub against the rail; the conical taper of the wheel tread naturally centers the axle, while the flange acts as a failsafe during curves and lateral shifts.