I would have to politely disagree with you there, LION
I just paged through the 1941 Locomotive Builders Cyclopedia and there was an index of over 150 locomotive parts suppliers in the back of the book.
Everything from staybolts to seatboxes, superheater headers to headlights there was a manufacturer, and a few competitors, to supply these parts.
Yes, the bigger roads did much of their own foundry and machine work. Altoona bult hundreds strike that: thousands of locomotives from the ground up.
But, if you were so inclined, you could buy complete boilers, (Vulcan Iron Works); frames, (Buckeye Steel Castings or Commonwealth); Driving wheels, boxes, running grar, (General Steel Castings, Franklin, Standard Steel Works) there were five manufacturers to choose from for pilot beams and front couplers.
Add to this partial list there were manufacturers for injectors, stokers, reverse gear, drawbars, boiler lagging, whistles, air pumps, speed recorders, valve pilot, low water alarms, thermic siphons, gauges and water glasses, flues, feed water heaters… the list goes on.
Prime Manufacturing Co. made cab windows and the little frames that the ICC inspection report was kept in.
I’m just pointing out that there were literally hundreds of manufacturers and you could “nearly” build a steam locomotive from a catalog of parts from outside manufacturers.
Anyone who is a student of the steam locomotive should avail themselves of at least one copy of a Simmons-Boardman Locomotive Cyclopedia. I have the 1941 edition which also covers early Diesels and has some beautiful color renderings of those classic paint schemes!
Happy railroading! Ed