I think it's a shame that there is currently no purpose to using higher quality locomotives along with the higher quality wagons improving the way that this mod currently offers, but I have an idea for how to address that.
I suggest that trains using higher quality locomotives and wagons should be able to travel faster over the rails, as well as having a higher acceleration power.
The acceleration power portion is simple enough to achieve, however the speed issue is (slightly) more difficult to address, because the max_speed variable must be adjusted on both wagons and locomotives.
I think that the same general rule of "30% better per quality level" that the wagons use is appropriate for both speed and acceleration power, so existing code shouldn't need too much modification to make that work.
I would much prefer this approach to higher speed trains rather than "adding a new locomotive that has a new recipe but can go faster" or "just boosting the max speed of existing trains".