Make minecarts be useful

Make minecarts so you can easily change the different speed of itself, on the latest snapshot of Java, you can do this with commands, but could it be a possibility to make an interface on the minecart itself to set a top speed, if a player wants to go faster they can just change it on the minecart UI like "Set up max speed of: 16" so they can go twice as fast when they go on a Redstone impulsed rails, same thing if they want to go slower, "Set up max speed of: 4" and they'll go slower, this could be really useful for mechanisms and farms, travel faster or make fun attractions.