Why heavy trains adopted diesel-electric traction

Did locomotives adopt diesel-electric drive because a suitable mechanical clutch and gearbox were impractical?

Heavy trains put severe strain on mechanical transmissions, and stopping power to change gears is a problem when starting a large load. Electric transmission supplies strong starting torque and smooth power without those stepped gear changes.

A mechanical gearbox can propel a railcar and its trailers because the load is relatively light. Starting a thousand-ton freight train places much greater strain on the transmission, and shifting gears interrupts the power needed to keep that load moving. Diesel-electric traction makes the locomotive a mobile generating plant with electric motors providing torque from rest. Hydraulic transmission offers another way to transmit the power. Some mechanical shunters can operate with a very limited choice of low gears because their work does not require the same combination of speed and load.