Helix/Spirals

Decide the maximum gradient for your trains and halve it to allow for curves friction. Decide how thick the baseboard will be and how much clearance you need between the stock and the bottom of the baseboard. That and the rail height will determine how many circuits you can get into the six inch rise. Knowing that and the gradient will determine the total length and diameter of the spiral. To keep the diameter down you could use a coil rather than spiral, where the diameter starts small and gradually increases by one track clearance (about 50mm.) each circuit. That is also easier to build using foam boards.
Ken.

I would not go smaller than 3rd radius curves. Mine is just over and older DE models struggle compared with more modern Heljan, Bachmann and Hornby. Of the three brands the Hornby 50 is the only one that does not stall with longest train possible on the curve. The gradient works out at @ 1 in 30.
Chris

In message , Keith J Patrick writes I have a helix on my garden railway, and it is 4 foot radius. My 0-6-2Ts have no trouble traversing this with 40 goods wagons.

Helixes have been used for several years over here to connect two levels of layout. Experience shows that in HO/OO you need about 3" rise rail to rail to allow for the substructure. It's the grade of one turn of the helix that counts. Also, you need at least 1-1/2" from track center line to nearest obstruction. The following table will tell you more (numbers rounded off):
Radius Length 1 turn Grade Diameter of helix 15" 94" 3.2% (1 in 31) 33" 18" 113" 2.7% (1 in 37) 39" 20" 125" 2.4% (1 in 42) 43" 24" 151" 2% (1 in 50) 51" 30" 188" 1.6% (1 in 63) 63" 36" 226" 1.3% (1 in 77) 75"
Keep in mind that a curve adds drag. IMO, an 0-6-2T trailing 20 or so wagons will have some difficulty on a curving grade greater than about 2.5%.
HTH