When you accelerate on an Iron Fence and a Magnetiser is coming, you can’t pass it. This is because the fence is smaller than an actual block. Because of that, the highest possible Magnetiser-ladder is 20. Therefore it is pretty much useless. When it would have the size of one block, the speed itself would grant that you can pass the magnetiser (which gives you no further speed) and use the next 20 Iron Fences. Which that improvement you can travel from the bottom of the map to the top really fast.
Greez BlackBone