Create a powered track 50 segments long, with sufficient redstone so that all sections are powered. This will allow a minecart to reach full speed quickly. Then at the end use unpowered rail to create a diagonal section - as long as you like. Send a minecart along the powered section towards the diagonal section. It will not slow down and will go apparently as far as you can be bothered to lay track. I have created a track of approx 1,200 unpowered rails laid diagonally and the minecart had no problems getting to the end and falling off.