For the record, I found how to get the classic cart behaviour back. It's the carts-old mod, found at
viewtopic.php?t=2451. You can only use up to version 0.4.14 of minetest to run this mod.
Again, what I like about the classic cart behaviour is its simplicity - you get on the cart and it automaticaly starts to ride. You get off the cart it stops riding. Briliant simplicity.
With the newer implementation of carts mod, in addition to the normal rails you also need to craft power rails, or else padling you won't go far and you cannot enjoy the view (since you have to padle facing down). I don't get what is fun about the newer implementation of carts. What does it bring that it's new to gameplay? In my opinion it is more of a pain in the neck having to craft power rails, brake rails, mase fragments...