What is the difference between OO9 and HOe gauges?

The confusion which arises from this question is due to the fact that both Europe and the United Kingdom use subtly different systems for their respective model railway gauges. Europe use the HO gauge, which is equivalent to 1:87 scale. Conversely, the United Kingdom adopted OO gauge as their standard model railway gauge. This equates to a scale of 1:76.2.

Both OO9 and HOe gauges were designed to model narrow-gauge railways. One of the difficulties inherent in modelling narrow gauge railways is that they all tend to be unique, with a specific set of requirements and operating environments. The term 'narrow gauge' is a very broad parish and generally refers to gauges that are less than the standard gauge of 4 ft 8 1/2 in (1,435 mm). As such, they tend to have a variety of gauges which lie between 1 ft 11 5/8 in (600 mm) and 3ft 6in (1,067 mm).

Modelling this variety of gauges could lead to all sorts of confusion in the scale modelling world, so a compromise was adopted. Both the European and UK systems opted to adopt a track gauge of 9 mm for their respective narrow-gauge systems. This made a lot of sense as it is based on N gauge so inevitably made production easier for manufacturers.

Effectively the European HOe system is based on HO gauge and its base scale of 1:87, whereas OO9 gauge, as the name suggests was based on the UK's OO gauge and its base scale of 1:76.2 scale. Both adopted a working track gauge of 9mm. So to all intents and purposes, HOe and 009 gauges are essentially reflections of differing national systems, whilst being completely interoperable.