The Chessie System, for the most part, did not believe in 6 axle engines. They bought GP35's 38's and 40's by the hundreds. The merger of Chessie and Seaboard to from CSX happened Nov. 1, 1980. At one time the mother-slug combination also used the fuel tank on the slug to supplement the fuel tank on the mother and thus extend the time between refueling - that experiment failed. A similar experiment was tried on the Seminole Coal Train that operated between Chattachoochee, FL and the power plant at Bostwick, FL. In the early 90's power for the train was two GE Dash-8's, a 30000 gallon tank car was connected between the units and was configured to supply both - with this configuration the train was refueled weekly not every other trip - it was stopped when it was determined that the tank car could not withstand the repeated buff and draft forces it was experiencing in the service.
When the mother-slug gets used in road service (and they do from time to time) the locomotive set is limited by its horsepower to maintaining about 25 MPH - the speed at which the mother stop supplying electrical power to the slug. That speed issue is why mother-slugs are not normally used in 'real' road service, the use of the electrical power below 25 MPH and thus the tractive effort of TWO powered carbodies is what makes their use effective in yard switching and road switcher type service.
