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):

• 3-rail short equipment: 31 inches (O-31)
• Modern long cars: 36-72 inches
• Large brass / scale: 72+ inches

G scale (1:22.5): typically 4-8 ft radius for outdoor garden railways.

S scale (1:64): 22-30 inches typical.

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 mainlines never use less than ~2 degrees curvature, equivalent to ~600 scale ft radius — far beyond hobbyist’s space

Conversion: scale degrees to model feet: A “30-degree” curve in HO ≈ 30 inch radius — easy formula to remember.