This has just come to me, what if there was Powered Rails that are capable of a higher speed. You would need a different type of rail and cart dependant on the level speed.
Powered Rail
Refined Iron = Iron
Copper Ingot = Uninsulated Cable (ie Copper, Gold, Iron)
Glass Fibre = Stick
Empty Cell = Blank
BatBox = Power Storage
Powered Cart
Level 1 - Copper
10x Minecart Speed
Level 2 - Gold
20x Minecart Speed
Level 3 - Iron
50x Minecart Speed