Wire

[Leslie BT]

H0 or 3.5 mm   1:87   16.5 mm (0.65 in)   H0 scale was introduced in Britain in the 1920s, and although it stayed as the most common worldwide modelling scale, in Britain H0 has little commercial availability and is generally only used to model the British prototype by a small number of modellers.

00 or 4 mm   1:76   16.5 mm (0.65 in)   The most popular railway modelling scale in Britain. The correct track gauge at the scale of 4 mm per foot should be approximately 18.8 mm. At 16.5 mm, it has a track gauge error of approximately -12.4%.

[Colonel Kilgore]

Ah - so naughty Airfix were fudging things and suggesting that their c. 1:72 figures could work with both HO and OO....?

[Smoking gun]

Leslie and Simon,
The British had problems developing their model railways the "loading gauge" (how big, width and height) for rolling stock was smaller than European and US loading gauges. The electric motor technology of the time didn't allow sufficiently powerful motors to be installed in correctly sized bodies. So the UK scales are N = 1:144, TT = 1:100, OO = 1:76 as above, and O is 1/43. O is the scale of Dinky cars.

UK models do look slightly "narrow gauge" when viewed head on because the bodies are too wide for the for the axle width, there are several fine scale standards in the UK for OO and N model railways but the older standards are still used for off the shelf, ready to run, models.

Best wishes,
Martin Buck