Radius = 47.7 "

The math: circumference of a circle (the trip around the helix) has to rise no more than 1" in 100" for 1% grade. So a rise of 300 inches is required for 3". Pi (3.14) x D = circumference. 300/3.14 = 95.54. Divide this by 2 to get the radius.

Espee over Donner

Radius of 48 inches

is pretty big for most spaces. That's probably why most folks use closer to 2% for a helix? .......DaveB

Keep in mind roadbed thickness is not zero

Another way to fit in a helix

Another way to fit in a helix is to realize it does not have to be circular. For example a 4 inch rise should accommodate a reasonable thickness of road bed and provide more than the minimum clearance. At 1% that grade would require 400 inches of run, 2% would be 200 inches of run. A radius of 30 inches will produce 188.4 inches of run for a circle of track. Adding about six inches of straight track between the halves will get you to 2% adding 106 inches to each side will get you to about 1%, that is 8 feet 10 inches per side.

Changing heights of levels will save some and going to a larger radius will save some as well. One must choose wisely. I would consider minimum radius first and look at what can be done scenically with the space that takes up.