The E20 and E21 locomotives became EP-4 engines to be used for passenger service. They were modified before delivery to remove driving controls and windows at one end to permit moving new, improved main circuit breakers into a cooler environment. The Milwaukee Shops replaced the operating controls in the "B" end with a steam generator before they entered service. The loss of this cab was operationally inconsequential, as many Milwaukee electric locomotives were normally turned at the end of their runs in Avery, Deer Lodge or Harlowton, the road having preferred to maintain only one set of controls even on double-ended units. The most important and final major modification was the provision of multiple unit controls for trailing diesel-electric locomotives. This system was designed in-house. It was not uncommon to see several diesel-electric locomotives being led by, and controlled from, one or two Joes (or a set of Boxcabs) in the 1960s and 70s.