It's also how far apart they want to have the supports.
For example, on Millennium Force's lift and camel back hills, there is 4-tube track because tall supports cost a lot of money, and if they can spread out the supports and use less supports, then they save money. (Yes that was grammatically correct.) Millennium Force switches to triangular track when going through the tunnels and on the low section of track before the final turn.
Then at the opposite side of the spectrum, you have the Superman Ride of Steel Treo which all have 2-tube track for some forceful parts, but have loads of supports. They probably did this because the manufacturing of the intricate cross ties and extra tube bending was more money than having a bunch of supports.
This is the same reason that Intimidator 305 has a double spine. The supports are very far apart....not to mention the lift hill.
It also true about the forces, which is why Millennium Force has square track for its final over bank, even though it is low to the ground.
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