They were fitted with multiple-unit train control systems, and could thus be joined together into larger sets and operated from a single control station. They were also retrofitted with a special multiple unit control system designed by an electrical engineer of The Milwaukee Road. This enabled the crew of a boxcab to control trailing diesel electric locomotives. 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.