Auto-Train Corporation began service on December 6, 1971 between Lorton, Virginia and Sanford, Florida. The service was a big hit with travelers. Auto-Train expanded and established a short-lived service between Louisville, Kentucky and Sanford, Florida. Lack of success from the Louisville expansion, high crew costs and several spectacular accidents put the company into bankruptcy and ended its services in late April, 1981.

The SCL was in need of replacement power for its Bone Valley (near Tampa, Florida) phosphate lines, and GE suggested road slugs for this operation. A MATE differs from ordinary slugs in three ways: their 65,413 to 67,588 pounds of added tractive effort surpasses that of most slugs, they are designe to operate at road speeds (they are equipped with Blomberg trucks with GE 752 traction motors), and they also serve as fuel tenders. Each MATE carries 3250 gallons of fuel+that the modified U36B can draw on while under way. These remain the only road slugs constructed by a major builder, but that could change. The first 10 MATE's (originally SCL #3200-3209, later SBD #5200-5209) were single ended, and could onl take power from the mother at one end. The second batch (SCL #3210-3224, SBD #5210-5224) are double ended, and can take power from either end, or in a third mode of operation, can be placed between two mothers and draw power from both. The double-ended MATE's have headlights and numberboards at both ends, the single-ended MATE's have a headlimht and numberboards at one end only.