For this update, minecart physics has apparently been reverted, however, stacked minecarts with tnt do not explode with the same force as they used to. The pictures taken at the same altitude show the difference, the larger explosion is from before the 1.8 pre releases and the smaller one from after. The stacking was achieved by having a dispenser place 27 minecarts with tnt on an powered rail, then powering the powered rail thus sending the minecarts off the rail (as they are stacked they keep velocity). The minecarts detonated upon impact with a dirt block. Note: I believe it is a useful feature to keep the large explosion radius as it provides more opportunities for vanilla war servers.