The design was highly successful, replacing a much larger number of steam locomotives, cutting costs and improving schedules.

They were also retrofitted with a special multiple unit control system designed by an electrical engineer of The Milwaukee Road.

However, the EF-4 "Little Joes", which were also retrofitted, were more often seen leading diesel electrics than boxcabs, which had by then been largely relegated to the role of helper or bank engine.

In the 1930s, with train tonnage increasing, the Milwaukee began to create three-unit locomotives out of the boxcab units.

The center units were therefore shortened by removing the cab, the pilot truck and the frame extension; these were nicknamed "bobtails".

In 1951, five four-unit locomotives were created; these were classified EF-5 and could have any combination of regular or bobtail units in the center two positions.

In 1953, two EF-1 locomotives, E69A/B and E28A/B were converted back into passenger service as class EP-1A to help the aging Bi-Polars on the Puget Sound Extension.

During the rebuilding process, they received steam generators, engine trucks from scrapped F7 Hudsons, and high speed gearing.

They were later repainted into the Union Pacific Railroad scheme of Armour Yellow with Harbor Mist Gray roofs, and red dividing lines.

Unit E50A+B, originally 10200A+B, the first locomotive built, still survives and is preserved at the Lake Superior Railroad Museum in Duluth, Minnesota.