One thing for the OP to keep in mind is that there are a lot of model trains that have no real-world prototype: you can get models in the Santa Fe "Warbonnet" scheme of locomoties Santa Fe never owned, and the multitude of "Dockside" 0-4-0 models trundling about come in a plethora of roadnames other than the one railroad that used the actual locomotive. And, of course, many model locomotives aren't based on any particular prototype, but are instead intended to be a "typical" type that don't represent any particular locomotive. You won't find prototype photos of those locos, because of course they don't exist.
How I do it: I start with photos of the railroad I'm modeling, and use the photos to drive my modeling, rather than the other way around. I'm no great shakes as a modeler/scratchbuilder, but I do it like this: First, I pick a locomotive to model, and look for documentation of that locomotive--from books, photo collections, or, in some unique cases, if the locomotive is still around I can visit it. Photos and other examination will tell me if there is anything unusual or distinctive about that particular engine as opposed to others of its type. Railroad museums and archives are useful for this purpose. Thanks to the World Wide Web, in addition to railfan sites there are also academic/history sites which can provide useful looks at locomotives, as well as stations, industries, and other things that often don't get enough direct attention from railfans trying to get a photo of the engine. But sometimes it's just really nice to be able to get a close look at an actual photo, instead of just a JPEG.
Next, I look for commercial models similar to what I'm trying to model. Is the commercial model close? If not, what modifications do I have to make to get it to match?
When it comes to actually modifying a locomotive, I am definitely okay with "close enough" rather than precise modeling. I'm not out to impress the micrometer crowd, I just want something that looks good enough to pass muster at casual inspection. I try to get the paint job right, adding details that will carry the flavor of the locomotive (like air horns, lighted number boxes, etc) but I don't worry about exact matches for paint color or precise detail matches. Close counts in horseshoes, hand grenades, and scale modeling that isn't intended for contests.