This bug seems to compound the negative effects of a previous bug in which powered carts will randomly launch a minecart forward at high speed. Right now, I am working on a train station which involves 10 carts being pushed by a powered cart. Due to the previously mentioned bug, I put unpowered rails along key points in the track as brakes to hold the train together. Now however, minecarts, (if going fast enough) seem to ignore the brakes. They slow down, and pass over the rail, then speed back up again like it wasn't even there. As far as I can tell, an unpowered rail is actually now incapable of stopping a minecart if it's given enough initial momentum.