Here is the first cut:
forget about railroad blueprints. They are for folks with barns - generally 1/4 scale, expensive, detailed, and generally unwieldy. Many, many artists have distilled them down to model-size drawings in things called Cyclopediae.
For this project, I have a copy of the MRR drawing of a Challenger from November 1959. It says th boiler barrel is 7 3/8" long, tapering from a diameter of 2 3/16" at one end to 2 5/16" at the other.
As you know, circumference increases quite a bit faster than diameter, so for a 5/16" increase in diameter you are looking at a gap of over 1".
More in a bit.
