🔄 Curve Radius Minimum Reference Chart

How a radius is measured and what it costs in space

Radius runs from the centre of the arc to the centreline of the track, so a half-circle eats twice that plus roadbed:

board width for a 180 degree turn = 2 x radius + roadbed width + margin
18 in radius  ->  36 in + about 4 in  ->  a 40 in wide board

prototype equivalent = model radius x scale ratio
18 in x 87.1 (HO) = 1,568 in = 131 ft, roughly a 44 degree curve

Main lines seldom exceed three degrees, near a 1,900 ft radius, so every model curve is a compromise.

The minimum belongs to the equipment, not the scale

Chart figures describe what typical short rolling stock will take. Individual models carry their own published minimum, and that governs. A workable check is three to four times your longest car: an 85 ft passenger car is 11.7 in long in HO, which puts it on 24 in curves or broader.

• Check what a number names. HO and N sectional track is sold by radius; three-rail O tinplate is sold by circle diameter, so O-27 track lays a 13.5 in radius.
• Scale ratios are not interchangeable. HO is 1:87.1 and N is 1:160, but O varies by country - 1:48 in the United States, 1:45 in much of Europe and 1:43.5 in Britain.
• Widen track centres on curves. Long cars overhang inward at the middle and outward at the ends, so tracks that clear on straight track can sideswipe on a curve.

Frequently asked questions