To David's thought and Bill's statement, there is no forced draft in a
locomotive unlike a fan driven HVAC unit. You create a draft, or negative
pressure zone, in the smokebox by introducing steam or by cylinder exhaust
steam. Because of that you want a minimum pressure reduction through the
flues and tubes to enhance combustion in the firebox. As to tube material
I've always used fully annealed seamless tubing. One of the advantages of
seamless is that you reduce the chance of the seams splitting when rolling
the tubes into the sheet.