I experimented with some minecarts in a private world and released that sometimes the minecarts go through eachother. I have a old world where I use this mechanic for redstone that the minecarts colliade with eachother and and now it only does work sometimes. It doesnˈt matter if an entity is in the minecart or not it can be better reproduced if there is some lag in the world.
A video with the reproduction of the bug: https://1drv.ms/v/s!AknKLCEsl1RcguYnQq71-QYxtQSRnw?e=2h13D2
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It also happens when Minecarts move at normal speeds and this didn't happen in 1.19/1.20.01 this bug breaks my builds.
Can confirm, affects 1.20.51. Although, I couldn't reproduce this on normal rails/with minecarts going at normal speed, so this seems to mostly affect minecarts travelling at higher speeds.
Steps to reproduce:
1. Build a short track using only powered rails (like the one shown in the video above).
2. Activate all the rails using a redstone torch/es.
3. Place a minecart at either end of the track.
4. Make sure both minecarts intersect, and that they're both going opposite directions of one another.
Observed results:
The minecarts won't always collide with one another, and will phase through one another when travelling at higher speeds.
Expected results:
Minecarts should be able to collide with one another at higher speeds.
In general, any mob that is moving fast enough can move through another mob. Minecarts do have a repelling behavior toward other minecarts in addition to normal collision, but it also seems to work only at slow speeds. I expect this behavior is intended because it approximates real-world physics in which faster-moving objects have more force and therefore are harder to stop.
About Minecart repelling specificaly, I would guess that they have a small range where they sense other minecarts outside of them, and that either (1) a minecart moving a full speed moves farther in one tick than the width of the sensing range, or (2) the repelling is only triggered if the other minecart is within the sensing zone for two ticks in a row. Either way, I expect it has been designed to trigger bouncing of slow carts and pass-through of full speed carts. You can design around that by adjusting the size and shape of your rails and the proportion of powered and normal rails.