Diagrams For Scratch-Building

The answer to your question can vary widely.  If the railroad you model has a historical society, that can be a good (but, possibly expensive) first source of information.  Some locomotive diagrams / drawings have been published in railroad (model & real) magazines over the years.  Finally, some loco builders' diagrams and drawings have been published in book form.  You may not find info specific to the particular model you wish to build, but sometimes a diagram / drawing from the same "family" of locos will give you enough info to proceed.

Chuck

Just to add a wrinkle, with many locomotives, there is the "as built" state versus the "as retired" state. Many locomotives, even those staying on the same road their entire lives, go through many modifications over the years. Three recent cases in point are the UP844, UP3985, and UP4014 which all started as coal fired, were retired, then restored as oil fired, two of them changing color a couple of times. Many steam locomotives switched from coal-fired to oil-fired. Also applies to diesels (nose jobs were popular), especially the earlier ones. Many ATSF F3's and F7's ended their service looking like Geeps.

Generally major dimensions did not change on locos.    The B&O did build some of their own 4-8-2s that started life as 2-8-0s but that was a major rebuild.   The santa fe creating CF7s from F  units was another example of a major rebuild    In my opinion those kinds of rebuilds tended to be more rare.

I think it was much more common for the details and appliances to change.    For example on F units on the pennsy, they started life with doors over the front pilots and standard EMD grab irons - and single chime horns.    Over the years lots of grab irons were added to the nose to get to clean the windshield, the pilot doors were removed, the horns replaced and real different look, the skirting over the fuel tanks was removed.    However, the basic shape and dimensions were the same.   Pennsy steamers were the same way.    the K4s for example started with the headlights on the front edge of the smoke box and slatted pilots and manual reverse and no automatic train control and probably some other stuff.    Over the years they got stokers, power reverse, the "beauty treatment" that moved the headlight to the top of the smoke box and the generator moved to shelf where the headlight had been.   Another platfrom was added to the front of the smokebox for a maintenance guy to stand on to service the generator.    Finally that last modernization included added a large cast steel pilot that was slight steamlined looking.    As far as I know, none of the 475 were ever converted to oil.    A few Mikados were.

So you start with a drawing to get the dimensions for the basic model.   Then you probably want to get photos of the loco at the stage in life  you want to model it, to see what various details were at that point.

I have a library of car and locomotive cylcopedias issued over the years in many formats that have detailed and dimensioned drawings of car prototypes.  I find them a really valuable resource in my modeling and many can be found online on the popular auction sites. 

You might look for back issues of "Mainline Modeler", an out of print Mag that published the kind of stuff you are looking for.

ECI

A good example I thought of was the ATSF #3450 series. These started as Pacifics. There were modifications made to the firebox as I was told and the rear support truck was changed to 2-axle. They looked like Hudsons and were often referred to that class because of the wheel arrangement, but ATSF referred to them as Pacifics. The 3460 series were built as Hudsons, the first being streamlined (the only one in the ATSF fleet). The others were unshrouded. Unfortunately, 3460 went to the torch early on. I think 3463 is the lone survivor and is resting in a park.

More of a diesel reference source, when I was working on my ex-ROCK Mopac GP38-2 and Frisco GP38AC projects, I found these diagrams helpful:

https://trainiax.net/index.php