As originally tested, the Milwaukee was not impressed with these locomotives, finding them prone to wheelslip. The World War I-vintage General Electric motor-generator substations had difficulty supplying more than two EF-4s under heavy load, which meant that their true ability could not be demonstrated. Additionally, the controls were initially labelled in Russian. After being modified with increased weight, raising the maximum height of the pantographs and being provided with adequate power, the EF-4s were excellent performers and very reliable. Some substations were later modified to supply up to 3,400 volts to take advantage of the high power of these locomotives.