Despite raising the minecartMaxSpeed using the gamerules, the minecart's downward velocity remains capped. This can be seen when trying to ride a fast minecart downwards or when observing a derailed minecart fall (it falls much slower than other entities).

Steps to reproduce:

1. Build a vertically diagonal track with powered rails
2. Set gamerule minecartMaxSpeed to 1000
3. Enter the minecart
4. Compare the behaviour of the minecart going upwards and downwards

Observed result:
The minecart hits a speed barrier several times when going downwards:
riding_upwards_vs_downwards.mp4
In my testing at maxSpeed 1000 going upwards from bedrock to world height takes 9 seconds but going downwards takes 20 seconds due constantly hitting the speed barrier.

At even higher speeds (you first need to accelerate on a looped track) the minecart will instantly derail because it's moving horizontally much faster than it's allowed to move downwards

Expected result:
The minecart would respect the new speed limit when traveling downwards.