Just press the button and the mine cart starts! Great to use in train stations.
Put down 3 blocks in this pattern (It doesn't matter which kind of block you use). This is where the track will start.
Leave a 1 block space, then add 2 blocks upright.
Add two powered rails to the top of the block and leading down in a diagonal.
Add a button to the top block.
Add redstone between the two blocks to connect the button and the powered rail.
Add a mine cart to the first rail.
Connect as much track as you want to the powered rail. You're finished! Just get in the cart, press the button and you go!