If you set up a minecart like this, as long as it is within a certain distance that it will be able to hit the other carts even when slowed down, as long as there are carts on either side that have a block in front so they won't fall off, and is on a normal rail, when pushed into momentum it will continuously bounce back and forth off them, without changing it's momentum pattern.

The example I have of it below is an 8-block length of rail, but technically 6 for what the minecart in the middle travels. Have not tested yet with longer distances, but I know this would not work with the same length of rails requiring redstone as they bring carts almost to a dead stop- it could perhaps be reproduced in a smaller length though- (except activated powered rails, which could possibly have the same effect, but would be more likely to have all three bumping around with the burst of speed it gives).