I understand the logic-
Lighter carts go further than heavier ones.
Lighter carts can corner at higher speeds than heavier ones, got it.
Up hill slows them down, down hill speeds them up, got it.
But this change has made working with mine carts near impossible (This is a bug so don't ask me to post this on the suggestions forum)
I understand the change and I like the idea in theory, but going from "The mine cart stays on the track or it doesn't" (prior to the change) to "Well, it might stay on the track if it's slow enough, or light enough, but a heavier one won't even have the speed to get to the corner without boosters, which will send medium weight carts flying"....
I'm guessing (tested) there are 16 different cart weight levels based on their % of inventory filled. I'm also guessing speed (not tested) is split up into 16 different levels.
That's 256 possible combinations of weight and speed to account for.
And, all this while still magically expecting a mine cart to immediately make a 90 degree turn or switch to a 45 degree incline or decline at any speed.

The games great, but either leave the basic mine cart physics in place or give us a bunch of new blocks to work with so we can actually use them again. Banked corners, breaking rails (slow but don't stop), redstone carts that will maintain the needed speed, something!