Steam & Excursion > rebuilding steam engines

I've been researching steam engines for work and since my favorite road that used them is the NP I've learned a couple things I didn't know but also have more questions.

NP had 150 2-6-2 locos in the T class.  Details can be seen here:
http://www.steamlocomotive.com/prairie/?page=np

Some were rebuilt to T-1 class and kept the same wheel arrangement but 10 others were rebuilt to 2-8-2 W-4s.  How do you do that?  Everything is changed, wheel base, weight, firebox area, cylinder size.  What makes it the same locomotive?  With those changes, it  seems like it would have a new frame and boiler.  Even the T-1 had changes in wheel base, grate area and wheelbase.  
http://www.steamlocomotive.com/mikado/?page=np

They could have added a course or two to the boiler and a new firebox and possibly extended the fabricated frame. It's possible that they had to reuse a certain percentage of the original locomotive in order for it to qualify as a "rebuild." The Reading "rebuilt" some heavy 2-8-0's into 4-8-4's by adding boiler courses and completely new running gear and tenders. Pretty sure the same thing applied to diesels: A few RR's (GN and M&StL?) traded in FT's on 1350-hp GP9's, possibly because re-using the FT generator and other parts made the Geeps qualify as "rebuilds" rather than new locomotives.



Edited 3 time(s). Last edit at 10/28/16 09:30 by CPR_4000.