Factory-built slugs are rare: I only know of two groups (CP yard slugs from MLW in 1950s, CN ?hump? slugs from GMD in the ?1970s?) besides the SCL GE-built MATEs. I assume this means that most railroads have decided they are not economically worth-while. I'd love to know more about what SCL's thinking was in ordering MATEs. Note that some of their MATEs were single-ended, so a single U36B+MATE was the operational equivalent of a pair of U18B (or a pair of late first generation units: GP-9 or RS-11). I think this sort of combination was used in low-speed "Bone Valley" phosphate service, and may have seemed like the best way to get the efficiencies of second-generation power for this low-speed service.
What might help in figuring out what the thinking was would be some comparison prices: U36B, U18B, MATE. Anybody have anything?
My "guesstimate" was figured on the basis:
-- Two U36C would cost the equivalent of 120% of two U36B (based on a remark in an old "Trains" article)
-- A MATE would have, in comparison with a U36B, (i) no engine cost (ii) a relatively small proportion of the electrical cost (two motors, no generator, no control system or very simple one, some cabling) (iii) something less than full "locomotive mechanical portion" cost (frame, trucks, fueltank, drawgear..., but no cab and ?no air compressor?). So it ***might*** be under 40% of the price of a U36B (to justify ordering double-ended MATEs) or ***maybe*** less than the difference between the cost of a single U36B and that of two U18B (to justify single-ended MATEs).
But these are ROUGH APPROXIMATE GUESSTIMATED figures, maybe not even in a small enough "ballpark" for understanding SCL's thinking.