I saw on the mod page that "Less air resistance" was listed as a benefit of the higher tier locomotives and wagons, and I was taking a look at debug info to see just how significant that benefit was. The values I found surprised me. I might just not be looking at the right place, but from what I can tell the locomotives are correct (each one has .0025 less resistance than the tier before it). The mk2 wagon seems correct as well, being half of the vanilla wagon (same value as the mk2 loco). But the mk3 and 4 wagons have an air resistance value much less than their associated locomotives. And all the fluid wagons are all just using the vanilla air resistance values.

Considering that the higher tier locomotives have such high speed values compared to the base locomotive, these air resistance values probably don't actually impact gameplay much at all, but I wanted to bring these odd values to your attention.