From "The Electric Railway Journal" of 1924:

"While rail manufacturer is ready to lay the cause to modern traffic conditions, equipmentand air brakes, many engineers place it with the rolling of the rail and a satisfactort reason has not yet been offered." "The corrugation must be removed by grinding...........as promptly as possible after the corrugations manifest themselves, as once started the trouble rapidly grows worse."

In San Francisco I never noticed rail corrugations until PCC cars replaced the iron monsters. PCC cars have rubber mounted resilient wheels. Corrugations appeared where cars were braking for a stop where across the intersection no corrugations appeared in the rails.

MUNI had a rail grinder that had been built in 1912 by a Market Street Railway predecessor.