Definition of "counterbalancing"

"The ability of these large locomotives to run at such high speeds without pounding themselves to pieces", my correspondent concludes, "is a tribute to the vast improvement in the counterbalancing of American locomotives since a day in October, 1904, when one pair of the driving wheels of a Baldwin Atlantic type balanced compound lifted off the supported wheel of the St. Louis testing-plant when exerting no more than 320 h.p.

1964 March, Cecil J. Allen, “Locomotive Running Past and Present”, in Modern Railways, page 193