trainrider47 Wrote:
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> The
> firebox area is 132 X 75 1/4 but the grate area is
> 96 X 75 1/4. What accounts for the difference?
> There appears to be some sort of box about 3' back
> from the front of the firebox.
That's a wall built of fire brick, and marks the front end of the grates. What is in front of that is effectively part of the combustion chamber. Why do that instead of putting the throat sheet there, and having a longer traditional combustion chamber?
That type of construction was pretty common on articulateds where the front of the desired grate ended near the top of a driving wheel set. It is desirable to have the front of the mud ring fall between the wheels so it can be more easily supported by the frame.
Rio Grande's L-105's and L-131's boilers were set up this way, as are Union Pacific's 4-12-2's.
The fact that this drawing shows the front of the mud ring almost over another driver tells me that the boiler was cut and pasted on top of the two K-27 running gears.