Re: Doubling downhill

Since about 1900, most railroad's rule books were based on a national common standard. That is still in effect today, with most railroads east of the Missippi River using the same rulebook (General Code of Operating Rules - "GCOR").
On the other hand, the railroad industry's rules and proceedures for train handling and air brakes continues to vary, sometimes widely, between railroad companies. While the rules must incorporate any federal regulations and some obvious basics, everything else is up to the individual railroad's perogative. Each railroad's train handling proceedures reflects their own beliefs and attempts to address specific situations to their operations based on their experiences.
The rules will also vary as technology evolves. Origninally, the greatest stress was placed on slack control (generally using power braking). When diesel fuel prices went up, the rules changed to use more drifting and dynamics for fuel conservation. Some railroads would never allowed 12 axles of helper power on the rear, now common DPU practice.
Additional reasons to/not to doublehead downhill with steam:
Did railroads doublehead downhill when engines were equipped with a water or Sweeney brake?
Some rulebooks provide a whistle signal for another engine to assume control of air brakes. As early air pumps were notorious for failure, a second engine could take over control of the train while moving.
Does anyone know of steam locomotives being equipped with a main resevoir line to allow the second engine to assist the lead engine's air supply? This is standard practice for multiple unit diesels, but I'm not aware of it being applied to steam.
A second engine could use its engine/tender brakes to assist in controlling a downhill train. I wouldn't think this would be uused except in an emergency, since this could cause serious issues with slack control. Engine brakes were NOT left applied for long periods of time due to the possibilty of heating the tires enough to loosen them; I've seen engines equipped with water drip lines over the drivers to prevent this.