1) If it used the same tooling as this manufacturer
http://muleteamkits.com/
It was 1:67 which is within a whisker of S Scale (1:64)
2) They WEREN'T 20 mule teams ! They had 18 mules and 2 horses - Clydesdales or Percherons as the Wheelers. There were two mule skinners, a teamster (driver) usually rode the nigh wheeler and the swamper who road either the trailer (1st) or tender (2nd). Bringing up the tail was a 1200 gallon tank of water for the animals. Water for humans was in kegs lashed to the sides of the wagons. Total length from lead mules to tail of the tank was 180 feet and the tow weighed in at 60,000 lbs. I have seen the load for each wagon as 10 tons and for both as 25,
"The borax wagons, said to be the largest and strongest of their kind, were built in Mojave, California . The rear wheels were 7 feet high. The front wheels, 5 feet high. Each wheel had steel tires 8 inches wide and 1 inch thick. The spokes of split oak, measured 5 ½ inches wide at the hub and 4 inches wide at the point. The axles were made of solid steel bars 3 ½ inch square. The wagon beds were 16 feet long, 4 feet wide and 6 feet deep.
The two wagons held 25 tons, or a carload, of borax. Two of them, together with a trailer tank wagon that carried 1200 gallons of water, constituted a train. Each borax wagon weighed 7800 pounds and the combined weight of the two, loaded, (exclusive of hay, grain, and other provisions) was more than 60,000 pounds. However, there is no record that one of them ever broke down on the trail during the many years they were in service."
3) "These great mule teams traveled 162 miles from Furnace Creek in Death Valley to Mojave, California; and from the mines at Old Borate to Dagget, the nearest railroad points. Their routes carried them over some of the most forbidding land on the face of the earth.
There was not a single house or any other sign of habitation along the Death Valley trail. One stretch of 60 miles was without water. In the summer, temperatures ranged from 136 degrees to 150 degrees.
The 20 Mule Teams could cover from 16 to 18 miles a day. Camp was made on the desert floor each night. The one-way trip, from mine to railroad point, took about ten days."
4) "Both men were responsible for readying the team, feeding and watering of the mules, and any veterinary care or repairs that needed to be done. There was a mid-day stop to feed and water the mules in harness. The night stops had corrals and feed boxes for the mules. A day's travel averaged about 17 miles, varying slightly from leg to leg. It took about ten days to make a trip one way. Cabins were constructed by the company for use of drivers and swampers at the night stops.["
5) Twenty million pounds of borax was shipped by wagon before 1898 when the tracks reached the mines
6) Youtube has a bunch of videos showing a modern reconstruction being built and in action. Here's one
https://www.youtube.com/watch?v=jLSLqR6cAVo
7) Looks like a branch or short line (HOn3 anybody?) to a borax transload point would be interesting, although you'd have to scratch the wagon(s) and team(s) in HO. Of course, you could add a "Wild West" settlement around it giving you passenger and goods traffic.
8) Gotta do some research on how the ore was treated before rail shipment
9) Thanks for all the interested and informative responses to my original query !