A Wootten firebox is a type of firebox that was very wide to allow combustion of coal waste, sometimes known as "culm" or "boney". The low combusability of the boney coal meant that the fireboxes had to be much wider than a standard firebox. The firebox size meant that the locomotive crew rode the locomotives in camelback cabs mounted across the center of the locomotive boiler. The fireman was exposed to the weather as he fed the fire from the rear deck.

The following is taken from "The Engineer's Encyclopedia" by John G. Winton and Willam J. Millar, 1890, page cxxx:

The Wootten Firebox - One of the latest novelities in locomotive building has been achieved in rather an indirect manner by Mr. John E. Wootten, formerly Manager of the Philadelphia and Reading Railroad Company. It had occurred to Mr. Wootten that the enormous amount of slack or refuse coal, which is to he found around all coal mines, might possibly be utilized in locomotive fire-boxes, where the opportunity of an enormous draught is possible. He therefore patented a fire-box with a very large surface, indeed, so large that, whereas the fire-box in general use presents a surface of about twenty-six square feet between the wheels, Mr. Wootten, by lifting his fire-box above the wheels, was able to utilize a fire-box with about seventy-five square feet surface. There is a fire-brick arch or division, which is a very essential point in are design of the Wootten fire-box, and gives much of the success of the engines in getting the necessary draught for burning fine coal or slack. Besides the advantage that it gives of utilizing what was formerly worthless waste coal, these engines make steam freely, and haul the heavy express trains of the Union Pacific at a higher rate of speed than has ever before been attained on that road. The coal used is taken from the mines owned by the railroad, and is bituminous, though light, in its character. It is, however, successfully burned without any sparks, a result, of course, due to the enormmous grate area, while the heat radiated from the arch fire-bricks or wall maintains an even temperature and insures complete combustion. The large area of the grate prevents any appreciable lifting of the fire, and the small pieces of live coal that are sucked up by the blast are burned on their way to the flues, owing to the high temperature of the brick arch. In the Wootten express engine, of which we give an illustration, it will be seen from the prospective view of the engine and tender, that the engines have two cabs, and thus the fireman is more efficiently sheltered from the weather than is usual on other engines. The severe climate of Nebraska and Wyoming in winter necessitates a very efficient protection for the men working the engines, and the arrangement shown, we are told, is found to answer well. The engine referred to above is one of the large class built by the Rogers Locomotive Works, of Paterson, New Jersey, for the Union Pacific Railway, from the designs of Mr. Clement Hackney, Superintendent of Motive Power of that line.

Union Pacific owned its own coal mines at Rock Springs, and their use of Wootten fireboxes was a move to use the waste coal from those mines. After a very brief time, it became apparent that Rock Springs coal and Wootten fireboxes did not mix well and the locomotives were rebuilt to use standard fireboxes and standard cabs.

As noted in the quote above, UP's Wootten 4-4-0s had two cabs. One for the engineer that straddled the boiler, and a partial cab that protected the fireman on the rear deck.

Union Pacific operated 11 2-8-0 locomotives with Wootten fireboxes and camelback cabs, built by Baldwin in 1886. All 11 were rebuilt in 1893-1895 by UP at Omaha with standard fireboxes and standard cabs.

Union Pacific operated 10 4-4-0 locomotives with Wootten fireboxes and camelback cabs, built by Rogers in 1887. All 10 were rebuilt in 1891-1892 by UP at Omaha with standard fireboxes and standard cabs.