The main issue with vehicle diversity is improper timelining and no easy way to upgrade vehicles automatically, there shouldn't be more than 2 vehicles for transporting a given good at a time, preferably just one most of the time, but over the course of a 100 year game there could potentially be switches every 10-20 years (bringing incremental capacity/speed improvements), with timeline/show obsolete turned on you only see a small number of vehicles.
The only problem then becomes keeping track of all your vehicles and upgrading them over time of course. I think Raven, and certainly I had planned vehicle sets for pak64 which would follow these sort of timeline based incremental upgrades, there comes a point where you have to balance between the micromanagement of upgrading and the richness of changing vehicles though (usually by reducing upgrade frequency to 30 years, and then having the various good type vehicles overlap, so you upgrade passenger trains one year, then maybe coal next year etc.)
I've always preferred pak64 because the scale seems to be more consistent, pak128 often suffers from "sparse tiles", e.g. rail tiles seem to be too spacious (the tracks take up such a small bit of the tile), and the larger size exaggerates the inherent scale discrepancies in the game. It's quite cool that you can zoom pak64 up to 128 size now in software (it doesn't even look that bad xD)