Minecraft wiki entry for Minecarts, associated text: Unpowered rail at the start of a rail line with a non-transparent block next to it. Place a cart on it and apply a redstone signal to the block to launch the cart.
Powered rail at the start of a rail line with a block flush to the ground next to it, and an upward-facing piston under the block. When a cart is placed on the track, a redstone signal to the piston raises the block and to provide the initial push to start the cart moving.