>>9887254>>9887264>>9887329/lg/'s resident trainfag here, didn't see your posts when I posted
>>9887771 so I'll chime in now.
First off decide what scale you're going to be building your trains in. 6-wide is what Lego uses for its sets but tends to be too small for replicating actual trains. 7-wide is common among UK/Euro train builders, while 8-wide is most popular among US train builders. That's not to say you can't mix and match depending on what you're building, you just gotta find your preference. Pic related is an example of something in 6 wide, 7 wide, and 8 wide. Once you settle on a size you want your build(s) to be then you can move onto planning for everything to be consistent.
Second, for trains you're gonna need to utilize a lot of SNOT building in order to not end up with a box on wheels. Even something as simple as making a faux door by having it recessed in the wall by half a stud or so adds a bunch of detail to a model. Basically, try to think outside of the box more with your builds.