Powered Rail

The Power rail is a block added in Version 1.5. They are added to replace bug boosting but boosters still work as of Beta 1.5. It has 2 states, powered and unpowered. It was tweeted by Jeb first to prepare texture pack makers where it will be. The Powered Rail was implemented in the Beta 1.5 on April 19th, 2011.

Boosting Principle
When a minecart passes a powered powered rail, it will be boosted. If a powered rail is unpowered, it will stop the minecart. Powered rails act like redstone wire, and will pass the power to the next adjacent rail for 9 blocks from the source.

If a Powered rail is not connected to a active redstone dust, it will not act like a boost, but like a brake, and stop the Minecart quickly.

A powered rail boosts gives an occupied cart enough momentum to travel 128 tiles on a flat surface, or an unoccupied one 8 tiles. Having more than one powered rail next to each other doesn't increase the cart's momentum further, however each time the cart passes over a booster, the momentum is reset. This makes infinite loops possible, unlike with traditional boosters which does not reset the carts momentum. The optimal use of powered rails seems to be to put one every 32 blocks for maximum speed the entire track, or one every 64 blocks to get a speed that is still acceptable but doesn't use as much gold. If you have very little or no momentum to start with the powered rail will give a smaller boost, in this situation you can get up to maximum boost by putting two powered rails next to each other, this is the only known situation where having more than one powered rail next to each other is an advantage.

Gold usage seems quite reasonable this way, one stack of gold will give 2048 blocks of travel if you place a powered rail every 32 blocks or 4096 blocks if you place a powered rail every 64 blocks. If the track goes up a hill you will need to put the powered tracks closer together.

It's possible to get a powered rail to launch a minecart if you have a block on one side of the powered rail and the track going going out the other direction. Using that you can make it launch you when you push a button, as illustrated in this screenshot. Since you don't have any momentum to start with in this setup you will want to put two powered tracks next to each other.

A detector rail next to a powered rail will power the rail while going over it, so that way you can have them powered without it taking up any additional space. If you put the detector rail on one side of the powered rails it will mean that the powered rail will boost you if coming from one direction, but stop you if you're coming from the other, which is nice from preventing runaway carts going to far or on a multiplayer server to make sure people use the rail in the right direction. If you want the rail to be a 2-way rail you can put a detector rail on both sides of the powered rail, or simply place a redstone torch adjacent to the rail. Placing alternate powered and detector rails up a 1/1 slope will not propel a cart of any kind more than 3 blocks upward due to a lack of speed which is required to clear the booster before it returns to the "off" state. The cart will become stuck unless it is in a "train" of two or more carts, in which case the last cart in the train will become stuck.

Powered Rail Additional Properties
There are currently no curves for powered rails, however, regular rails still prefers to curve towards the powered rail. In cases such as these, the south-west rule applies.

A cart traveling on a powered rail that collides with an object (wall, single block, player, other cart) will reverse direction. If a track including powered rails is bordered by blocks acting as "buffers", the cart will indefinitely continue back and forth along the track. Having carts interact with each other on a short track designed this way can be used to chain multiple carts together as a "train". Once alligned, they will all move together at relatively the same speed.

A cart placed next to a solid object (stone, dirt, sand etc.) on a power rail without power, will start moving when the power is turned on. This makes it possible to easily remote start mine carts.

Trivia

 * Powered rails will always show as powered in the inventory, despite if you destroy it while it's unpowered.