4 mm scale

4 mm scale is the most popular model railway scale used in the United Kingdom. The term refers to the use of 4 millimeters on the model equating to a distance of 1 foot (305 mm) on the prototype (1:76.2). It is also used for Standard gauge

• • OO gauge uses 4 mm scale with 16.5 mm gauge track, which is inaccurately narrow since it is correct for HO scale (1:87.1). It is the most popular standard for 4 mm scale trains and is the only one produced by mass-market manufacturers. The traditional standard for wheels and track is a very coarse one with extremely oversize rails and flanges; in recent years, some manufacturers have switched to using the American National Model Railroad Association HO standard RP25 instead.
  • EM gauge uses an 18 mm track gauge, which is closer to accurate but not fully to scale. It was an early improvement on the standard OO system.
  • P4 gauge is an exact 4 mm:1 ft (305 mm) replica of real-life track and wheel dimensions. It is the most popular choice among finescale modellers working today.
• Narrow gauge
  • OO9 9 mm gauge (N gauge) track.
  • OOn3 12 mm gauge (3 foot, 914 mm) track.

OO gauge trains can be converted to work on EM or P4 by replacing or reprofiling wheels, fitting the correct width axles and moving the locomotive or carriage frames outward to suit. Products intended for the more dedicated modeller are generally designed to be buildable to fit any of the three standards.