Adding different Tiers of Minecart

The Java experiment for improving minecart speed is a good step but I believe a better way to implement speed changes would be via the minecart itself. Powered rails featuring gold proposes a crafting budget problem between more high speed rails or further unpowered rails. The use of copper or gold minecarts could change the speeds these carts go at and there properties. Gold is a very light metal so would move the fastest. Coppers electrical properties in real life could allow it to carry a charge for a certain amount of time after moving away from a redstone source. Iron could remain as the sole minecart with hoppers or chests so it maintains a viable use for users with it being lighter than copper but heavier than gold. Implementing other ideas about chaining minecarts together could allow for copper and gold chains to be added also.