OK, here we go.

The vast majority of locomotives have a single control stand on the right side of the cab, the right side being determined by which end of the locomotive is designated as the front. On locomotives equipped with conventional control stands (i.e., not desktop controls), a customer option is to have dual controls, which consist of placing a second control stand on the left side of the cab and orienting it so the controls are in their normal locations when operating the locomotive in reverse. With dual control stands, available cab floor space is reduced, but there's still sufficient room for walking. Southern Pacific wasn't a big user of dual controls, having just 25 (if I've counted correctely; WAF, feel free to help me out) locomotives equipped with it.

A variation of the dual controls concept was the Duplex Controller. It incorporated all the functions of two separate control stands into one very large box in the middle of the cab floor that also included an electrical cabinet. Because a Duplex Controller took up so much room in the cab, floor space was at a premium. Another feature of a Duplex Controller was that there's a mechanical connection between the two throttle handles. Duplex Controllers were relatively rare; off hand, I can only think of Reading's SD45, Pennsylvania-Reading Seashore Lines GP38s, and a small number of Penn Central GP38s. Eureka Southern 30 and 31 had Duplex Controllers (32 and 33 had conventional dual controls); all four units were bought used.

And then there were the C-415s. Apparently Duplex Controllers were standard equipment on early productions units, which is why SP's 10 were built with them. The locomotives were never favorites of the Mechanical or Operating Departments, and were mostly hated by operating crews. The units were beset with mechanical and electrical problems for their entire lives; initially there were serious problems with the 8-cylinder engine that resulted in crankshaft failures, and the strangely-designed cooling water system tended to wreak havoc with the electricals, especially as the units got older and water leaks became more prevalent.

But it was the Duplex Controllers that the crews really hated. Because it took up most of the floor space, when passing through the cab going from one end of the unit to the other, one either had to climb over the engineer's seat or sit down on it and rotate 180 degrees.

Jim700 mentioned some of the reasons the SP&S (and I presume, BN) crews didn't care for their two. I'll just add that one of the issues SP crews had with their's was that during hot weather it was common to run with the cab doors open to get a little breeze. That was fine unless the unit was running backwards (i.e., with the radiator end leading), because when the radiator shutters would open up, the cab would fill with hot, moist air akin to a sauna.

SP's units spent their entire lives assigned to the Los Angeles Switcher Pool, with Taylor Roundhouse as their maintenance base. From delivery in late 1996 until around '72-'73, they mostly worked the locals out of City of Industry, usually in pairs or a single unit MUed with an SW1500. After that, they could also be found working the various Basin Haulers out of Taylor Yard and West Colton Yard, or were stored. All were retired between 1977 and 1979. The last unit to operate was the 2407, which was assigned to the "Shop Dinky" at Taylor Roundhouse. It even had a sign on it that said SHOP DINKY. The Dinky was a job that had an engineer and a laborer, and moved dead locomotives around the Taylor facility.

As trivia, within a couple of weeks after being delivered, the 2401 suffered a mechanical failure - it wasn't the only one to do so - and was returned to Alco for warrenty repairs. Alco took the opportunity to remove the Duplex Controller and install two separate control stands instead. My understanding of it is that Alco did it as a demonstration project and offered to do SP's other nine if SP would pay the cost of the conversion. SP declined, which is unfortunate because the crews were quite appreciative of the work done to the 2401.

Personally, I liked the units, because they were different and presented somewhat of a challenge, for the same reasons I liked the U25Bs. Crappy locomotives to be sure, but kinda fun to run.