Powered Rail

The Power rail is a block added in Version 1.5. It was added to replace bug boosting, but boosters still work as of Beta 1.5. It has 2 states, powered and unpowered. It was first tweeted by Jeb on April 11th, 2011 to prepare texture pack makers for the change, and released in Beta 1.5 on April 19th, 2011. Here is a video of a use for it.

Crafting
Powered Rails are made somewhat differently from normal rails, replacing the use of iron with gold, and adding redstone to the bottom.

Boosting Principle
When a minecart passes a 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.

A powered-rail boost gives an occupied cart enough momentum to travel 128 tiles on a flat surface, or an unoccupied cart 8 tiles. Having more than one powered rail next to each other increases the cart's momentum further, however you won't get much additional boost. 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.

A powered rail on a slope can act as a launch pad for a minecart. When unpowered it will keep the minecart in place, and when powered, gravity will pull your cart downward, triggering the boost.

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 convenient to prevent runaway carts from going too 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. It will not reverse direction if it collides with a half-block, such as stone slabs. 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, or even to build mine cart junctions.

Bugs
If a Powered rail is only powered by another Powered rail diagonally up or down, you can destroy the rail powering it and it will continue to look and act like it has power until the block is updated. This can be used to have little to no power sources in your track design and up to 2/3 less powered tracks for uphill parts. It acts as a normal rail on the slope but can still stop carts. This means it can be used to obtain the momentum required to be boosted by other powered rail to create a good boosting system, as seen here.

Trivia

 * Powered rails will always show as powered in the inventory even if destroyed and collected while it was unpowered.