Mike;

Quote - I guess you just have to consider each part and pick the most appropriate modeler.

 This is exactly right, and is as true for the guys with multi-million dollar render farms and Maya/3DS/Lightwave as it is with those of us that do this as a hobby.  Sub-division, bump mapping and displacement maps are all tricks that were developed to keep the poly count below the crash and burn memory thresholds (and speed rendering time of course.). For hard edged objects you can't beat the spline modeler.
  Not trying to insult anyone's intelligence, only throwing out ideas..  If I were doing your  loco. I'd probably do most of it in the spline modeler, except for the cab and the backhead of the boiler.  I would probably add the rivets on the smokebox, boiler, and firebox with displacement maps and probably model the staybolts as the 3d equivalent  of NBW castings (nut-bolt washer) with surface replication, but that would be me,  I tend to build 3d models as if they were here in the real world with me, and there are a thousands ways to do it. My other hobby is model railroading so I tend to get a little carried away.
   My standard plug for Mark's classes, you can't beat them, and I've tried most of the classes available on DVD/online.. 
           R/S
Patrick "Sully" Sullivan