Hi Mike,
As I recall correctly, the tonnage rating is based on the smaller of the two calculations.
First is the TE, which is usually the controlling factor. The second is the adhesion factor (which has to do with how slippery a locomotive is).
Rail was added by SV after the tenders were added and the tanks removed to increase the adhesion factor to be able to use more of the maximum TE the engines can produce.
The Erie Triplexes (2-8-8-8-2ts) were an attempted use of the weight of fuel and water in the tender to generate more absolute TE. However, as the locomotive used up its supply of fuel and water, the weight of the tender reduced and made the tender engine very slippery.
Same goes for the Southern Railway's "tractors" where engines from unused scrapped locomotives were put under tenders of other newer power. (resulting in very odd 2-8-2+4-6-0 and 2-8-2+2-8-0 wheel arrangements).
Although I am not well versed in the articulateds, I was reading about the PRR's Duplexes in the late 1930's and 1940's. Basically it was to allow an extra set of cylinders on the ridged frame driver set. So a 4-8-4 became the 4-4-4-4 T1 for passenger service. It lowered the dynamic forces by using smaller rods. The T1 was a very slippery locomotive with 80" drivers.
They problem of the slipperyness was partially corrected in the Q2 4-4-6-4. It had 69" drivers. It was to replace the PRR Northerns when most other railroads were going to 4-6-6-4 articulated Challenger types. The Q2 had poppet valves and had an ELECTRONIC controlled valve (butterfly?) that if one one engine slipped, the steam would be adjusted so that the other engine would get more steam. This is supposedly the first ELECTRONIC anti-slip application that most diesels now use today (it's great-great-great-grandfather or some such). Many believed that the PRR Duplex was the next evolution of steam like the Lima Superpower but dieselization was too far along for the new generation steam to have any effect.
If an anti-slip device had been used on the SV 2-6-6-2s, more of the TE could have been used without needing more weight on the drivers. Of course more weight on the drivers is still a better solution at the expense of a larger tare weight.
Doug vV
