Jay,
You are right there is no visible opening to the boiler when this locomotive is assembled. Will provide a pic of an assembled model later today. It is really a beautiful model. The only stock well running ten wheelers mass produced appear to be the GPM NP & C&NW types and the PRR G-5 made by Weaver, MTH, Max Gray, etc.
Which Canon motor do you use?
Thanks for the sage advice regarding the drive shaft coupling.
Rob