Minecraft wiki entry for Minecarts, associated text: A single powered rail on the flat ground against a stop block gives an occupied cart enough momentum to travel 80 rail tiles on a flat surface, or 8 tiles for an unoccupied cart (in Beta 1.5, this was 64 blocks and 8 blocks respectively). Tests show that putting several powered rails in a row has observable diminishing returns with each additional powered rail on how much farther a cart travels.[1] This implies that the momentum gained is smaller if the cart's speed is faster and vice versa.