The Minecart (with Furnace) Rework

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.

Speed bar
[ not moving _ -6 b/s _ -3 b/s _ defined speed _ +3 b/s _ + 6 b/s _ not moving ]
If the furnace minecart is mounted, a speed bar with a speedometer appears above the hotbar.

• When “forward” is pressed, the speedometer moves to the right.
• If there’s no input, the speedometer moves to the left.
• If the minecart runs over powered rails, it’s never slower than its “defined speed” (the middle of the speed bar gets locked). 

Minecart with furnace rework
The fuel defines the speed the minecart with furnace runs at. To run, it requires player input via chained minecarts or running over powered rails. If pushed, it moves a little, but not at its defined speed. A normal minecart has a defined speed of 8 b/s to not hurt any existing builds. 

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".

Examples:
lava: 20b/s
stick: 7b/s

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).