What are the rules for elevated rails?

[mrvn]

Elevated rails need a ramp to go up from ground, that much is clear. But then they can hang in the air for some time before they need a support pillar.

But the rail planner is pretty inconsistent about placing those support pillars. It frequently wants to place a pillar at the end of every curve or straight so creating a path where one wants to have it instead of where the rail planner wants to go pretty much floods the rails with supports.

So what are the rules for those pillars? How often is one needed on straights and curves? What pillars are needed for S curves?