Well, someone did talk to one of the developers via Twitter about coal-powered minecarts. Considering how Dinnerbone reworked boats, I'd assume 1.4 would be a reworking of carts... At least coal ones. I'd imagine they don't want to mess with minecarts too much since people like to use them with detector rails and such to make complex contraptions which might break if you mess with the system.
Although, they made it so holding directional keys in Minecarts allows you to move. It's just pretty slow, and not automated like powered rail. But that's not what you're getting at.
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