The TR6 was not just SW8s coupled together with one missing a cab, these had some very distinct differences.

Years ago I ran a TR4, the SW7 version of a cow and calf. As built, TR sets are draw-barred together. The calf cannot run without a cow. There are no inboard headlights or pilots as they were not needed. They did not have mu, though most of the mu electrical was needed except the mu plugs and mu headlight switch. In addition, the rear of the cab does not have the lower windows to allow the engineer to see a switchman on either of the rear steps. In addition to not having a cab, the calf does not have its inboard steps, a handbrake or batteries. The air brake system is very simple as the calf only has a through brake pipe, main reservoir equalizing and a brake cylinder equalizing line that operates a relay valve to apply the brakes on the calf.

Note that in addition to adding a rear headlight, photos show the 1100 also had the lower rear end cab window cut in but only on the engineer's side. And strangely a photo of the 1113 running with the 1153 has its lower cab windows blanked off. Other than a rear headlight and pilot, I don't believe there were too many modifications to the 1100-1103 to allow them to run as single units. I haven't seen any photos of front mu being added, so did these keep mu at the rear only or was it removed?

The calves on the other hand apparently did get couplers so they coupled run with another unit. There was no reason to add a headlight to the "cab" end since these never were fitted with front mu. My question is were the SW8s that ran with the calves converted to cows? The calves would have needed to have batteries, a handbrake and air brake control valve installed (probably not visible within the hood) or always be coupled to the same group of cows. The biggest change the SW8s would need to run with the calves would be a pair of battery cables unless the calves got their own batteries.

These did back to an era of very different thinking. EMD advertised these as a 1600 hp locomotive, just like the FT/3A-B-B-A were advertised as 5400 hp locomotives. The early B units on hood type locomotives were similar to the calves, they did not have batteries or full air brake equipment so were dependent upon their drawbar connected A unit. There was also a labor issue involved, as the unions were insisting that each "engine" have its own fireman. This was finally resolved and the railroads began to realize that they could add or subtract units to represent the actual power needed for a train. Most B units were divorced from their A units and retrofitted with couplers, batteries, full brake equipment and sometimes hostler controls to allow them to stand on their own. The units got rear couplers and a variety of rear headlight arrangements. Early EMDs were restricted to four units in a consist for field loop dynamic brake control, but this began to end by the GP9/SD9 era.

It should be noted that the various SP carbody type A-units (Fs, Es and PAs) did not have front mu. SP finally began ordering this as an option on the last orders and began retrofitting them on older orders. Only some of the six axle Baldwins were built with mu, a few being retrofitted. By the SD7/SD9/GP9 era, all SP roadswitchers were ordered with full mu but it wasn't until the C415/SW1500 orders that switcher type locomotives were consistently ordered with mu.

The SP TR6/SW8 class is also notable because they were ordered as and classified as road units, not switchers. While SP did have some Baldwin S12s and Alco S6s with mu, they were still classified as switchers though many had the full road light packages and train indicators to allow them to work branch jobs and mainline locals.

SP obviously considered the TR6 and SW8s as light road units, the only earlier "switcher" types similarly designated were the GE 70 tons. It is interesting that EMD demonstrated these on the Fallon and Mina branches. I think Mina at the time was using a 6-axle Baldwin, so maybe EMD was trying to convince SP that a TR6 was a better option. Was NWP using the six axle Baldwins before the TR6 sets? Speed was clearly not a consideration as SP would have ordered SW9/TR5 instead.