In the late thirties, the Rock Island operated the “Rocky Mountain Rocket” train that was separated at Limon, Colorado. A section with only three cars went to Colorado Springs and the operator wanted to get a special locomotive that would blend into the whole train before hauling the shorter section alone. The solution was an E6 B-section with a cab and other equipment needed to lead a train, but with a blunt end to fit into the aerodynamic appearance of the train.
Due to the lower power requirement of the three-car train section, only one 1,000-hp prime mover was installed. The remaining space was taken up by a baggage compartment. The two AB6 were used in this role until 1965. After this, a second engine was installed alongside with a generator that replaced the steam heating boiler. The second, unpowered truck was now also powered. In this configuration, they were used in suburban service in the Chicago area until the mid-seventies.
