General changes

Chained Minecarts
Minecarts (every version of it) can be chained together by applying iron or copper chain in the same way as you would leash boats together, forming a solid connection between each cart.

Powered rails
Powered rails do not longer determine the speed to 8 b/s; they run the minecarts at their speed without requiring player input. The speed is instead defined by the minecart.

Example 1: A minecart has a max. speed of 8 b/s. If it runs over a powered rail, it will run at 8 blocks per second without requiring player input.
Example 2: A minecart with furnace and a lava bucket as fuel has a max. speed of 19.5 b/s. If it runs over a powered rail, it will run at 19.5 b/s without requiring player input.

Minecart with Furnace

The fuel determines the speed the minecart with furnace runs at. Tu run, it requires player input via chained minecarts or running or powered rails. If pushed, it moves a litte, but not at its defined speed.

A new interface shows the player how fast the minecart with furnace can run with a given fuel.

Every fuel gets used up after 1600 ticks (1:20min).
If there is no fuel in it, the minecart with furnace is unable to move (unless it's pushed).

The speed of the fuel is closely related to its "smelts per one" number (for example, 1 Lava Bucket can smelt 100 items). The steps up or down to the next fuel from "smelts per one" are relatively the same size as the steps up or down from the "speed".

Example 1: lava 19.5b/s
Example 2: coal / charcoal 8b/s