Blue Liner, I tend to keep similarly scaled vehicles in their own neighborhoods or scenes. Most important to me is how the scales of the various cars and trucks I use (all were impulse buys) relate to the figures I use to narrate a scene. These photos will illustrate what I am saying.
For the locomotives, I figure they are so spectacular in and of themselves as they travel around the layout, they command their own unique attention regardless of how nearby figures or vehicles might relate to them, IMHO.
For example, I use Artista figures (1/48) and the Preiser 1/50 figures in and around vehicles that are 1/48 scale, and I utilize Preiser's 1/43 figures and Roy Baker's Railroad Shop figures with vehicles that are 1/43. Also, I have found Artista's seated figures to be particularly adaptable to being seated inside vehicles. If you warm their little arms up with your hands a bit, you can bend them a little to make them appear to take hold of the steering wheels a little better.
And in the last analysis, I trust my instincts concerning what "looks right" and appears to be in the proper proportion.
In this scene, a mixture of 1/43 figures seems properly overwhelmed by the scale trains around them...
Here, I was conscious of how the figures related to the scale of the truck as well as
how they related to the doorway of the building..
In this scene, I used Roy Baker's figures because I felt their scale, as it related to the doorways, details, and farm equipment, and in particular their attitude, suited the narrative of the scene...
In this vignette, it was the narrative, as well as how the figure coming from the red truck related to the truck, in size and purpose, that dictated the ingredients...
The doorway dictated my use of the (1/48) Artista figure...
Here, it was the scale of the building, esp. the door, as well as the details of the narrative element that determined the scale of the figures I used, not the scale of the boxcar nearby (on its hi-rail track.)
And in these two scenes, 1/43 vehicles were used w/ 1/43 figures...