Model Train Minimum Curve Radius Calculator

Minimum Curve Radius for Model Trains

The minimum curve radius is the tightest curve a piece of rolling stock can negotiate without derailing. The longer the car, the wider the radius needed.

General rule: Minimum radius (inches) ≈ Longest car length (inches) × 3 to 4

For reliable operation: 4× car length. For “tightest possible / yard moves only”: 2.5-3× car length.

Typical minimums by scale (for the most common car lengths):

HO scale (1:87): most common:

• 4-axle freight, short passenger: 18 inches (standard “Atlas-style” curve)
• 6-axle locos, 70-ft cars: 22 inches
• 80-ft+ passenger cars: 24-30 inches
• “Big steam” (2-10-2, 2-10-4): 30+ inches

N scale (1:160):

• Most modern stock: 9.75 inches
• Short freight, switchers: 11 inches operates everything
• Smooth running of big stuff: 14-16 inches

O scale (1:48), and read this one carefully: In 3-rail O the track is sold by diameter, not radius, which trips up nearly everyone coming from HO. “O-31” is a 31-inch diameter circle, so its radius is 15.5 inches. Halve the number before comparing it with anything on this page.

• O-27: 13.5 inch radius, short postwar-size equipment only
• O-31: 15.5 inch radius, the traditional starter circle
• O-42: 21 inch radius
• O-54: 27 inch radius, where most modern scale-length cars become happy
• O-72: 36 inch radius, full-length passenger and big steam

S scale (1:64): 20 inch radius is the American Flyer standard, and 27 to 30 inches runs modern equipment comfortably.

G scale (1:22.5): LGB sectional track is the reference: R1 is 600 mm (23.6 inch) radius, R2 is 780 mm (30.7 inch), R3 is 1195 mm (47 inch), R5 is 2320 mm (91 inch). Short four-wheel European stock is happy on R1. Long American rolling stock generally wants R3 or better.

Adjustments for special equipment:

• 3-axle trucks: +20% radius
• Articulated locomotives (2-8-8-0, etc.): up to 2× the listed minimum
• Brass scale models: often 1.5× the plastic equivalent
• Long passenger cars (heavyweights, smoothsides): always check the manufacturer’s stated min

Why looser curves matter:

• Slow speed only: tight minimums work
• Mainline running: use 1.5-2× the minimum for reliable operation
• Realism: prototype mainline curves are very gentle, often 1-2 degrees of curvature (a 2-degree curve is roughly a 2,865-ft radius). Even scaled down, that is far more space than most layouts have

Prototype to model radius: Divide the real-world radius by the scale ratio. A gentle 1,000-ft prototype curve becomes 1,000 × 12 ÷ 87 ≈ 138 inches in HO, which shows why full-size mainline curves rarely fit on a layout.