Steam engine question

[Sumpter250]

items that affect draft - air hole, grate area, the gap between the brick arch and the back wall of the firebox, diameter and number of tubes and flues (as noted by Sumpter), how flues packed with superheating elements affected it's ability to breathe and smoke box design (exhaust nozzle, spark arresting components, diameter of the lower end of the stack, the gap between it and the exhaust nozzle, the stack's internal shape).

All of these things have to be balanced to each other, with consideration given to type of fuel, oil/coal, type of coal, array of oil feed to the fire, etc., operating speed of the loco, boiler size, and more other things than I can think of at the time.
Firebox, tubes and flues, and stack.....these are about all that's needed when the loco is standing. Everything else comes into play when the loco is working. (Exhaust nozzle, spark arresting components, diameter of the lower end of the stack, the gap between it and the exhaust nozzle, the stack's internal shape), these things are critical to working performance.
The exhaust nozzle, shapes and directs the exhaust steam from the cylinders, up the center of the stack.
Stack diameter, shape, and distance above the exhaust nozzle, determine the "venturi effect", or the ability of the exhaust steam to "draw" the hot gasses from the firebox, through the tubes and flues, and up the stack.
The idea here is, simply, the harder the loco works, the more steam is blasted up through the stack, the faster the heat is pulled through the boiler, the faster steam can be made.
Superheating elements, and spark arresting components, create resistance to the flow of "heat", and have to be compensated for.
All of this may be interesting, but where models are concerned, is never seen, and rarely ever modeled, except where it shows externally, as in air intake above the fire, to reduce smoke.