MORE ON THE SPIRAL DESIGN

This is how my spiral is designed.  It is flat in the middle which makes it easy for a person to reach in and adjust any equipment that needs attention.  Trains must drop down into the spiral and climb out of it.  The steepest grade is in the middle where there is also the smallest radius. Trains climb out of the spiral on the outside which has a gentler gradient and also the curves here are of larger radius.

The smallest radius curve is the initial one which takes the train through just one 90-degree quarter-turn at my minimum radius of 20 inches or 508mm. Curves then step out at 2-inch [50.8mm] increments so the remaining radii are 22, 24, 26, 28 with one final quarter turn of 30-inches.

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The grade down into the spiral is 1-in-40 or 2.5%.  Trains can manage steeper grades, with smaller radius curves, so long as they are going DOWN.

The climb out is a gentler grade at 1-in-48 or 2.1% and of further benefit - the radius here is larger - a mix of 28 and 30-inch radius curves.

I have highlighted the 24-inch [609.6mm] radius curve here, to make it easier for viewers to understand how the spiral is constructed.  The curve is interrupted before its final quarter-turn by a two-inch straight.  This straight pushes the spiral to the right, sufficiently for the next larger radius to begin, and creates the 2-inch separation between the tracks.

Enthusiasts wishing to adopt this plan for storage will need to consider how they will keep the trains separated in this staging yard. One way is to divide the spiral into sections or blocks controlled by relays. [More on that shortly.]  Another way is to allow a set time interval to pass in order to keep the trains apart.  If this approach is chosen, then it is imperative that your locomotives are speed-matched to run at the same speed at a giving setting - say SPEED-STEP 7-of-28, or SPEED-STEP 30-of-128.