Minecart Improvements need to be enabled.

By waterlogging rails and letting the water flow to only one side, the minecart will be propelled in the -z or -x direction, depending on which axis the rail is orientated on

Steps to recreate:

Build a linear rail line and waterlog one or more of the rails in the middle as shown in the picture.

Make sure one side is blocked off such that the water only flows on one side of the rail, perpendicular to it.

Observed results:
The minecart, instead of remaining stationary, will always move towards the negative direction of the axis it's facing on, regardless of which side the water is flowing towards. For example a waterlogged rail line along the x axis will push the cart towards the negative x direction

Notice that this is different of what it used to be without the minecarts experiments enabled, the minecarts used to move in the positive direction.