A Question of Scale

By Andy Joel

This was something I was asked recently, and I thought it a good idea expanding on in case anyone else ever poses the question! Worth noting that the opinions expressed are mine, and may not represent those of the society.

The question of gauge is one you need to consider carefully before you commit one way or the other. Hopefully this article will at least make you aware of the issues, if not help you decide.


Scale and Gauge

There are two related terms, so we will quickly discuss them.

The scale of a model is the ratio between the size (linear, rather than area or volume) of an object and its model.

The gauge is simply the distance between the tracks.

That should be simple, but unfortunately in the early days of model railways, British manufacturers got the gauge wrong. This was because of the limitations of the technology; British rolling stock is built to a smaller loading gauge, so the models had to be to a slightly bigger scale to fit the motors available at the time. So we are in a situation where scale and gauge do not quite match.


OO Gauge

Scale: 1/76 (about 4 mm to 1 foot)
Gauge: 16.5 mm

The most popular gauge is OO (pronounced “double oh”), both at the club and nationally. To give an idea of how big that is, a coach will be around a foot long, maybe a little less for older coaches. If you have a train that is four coaches and a tank engine, then it is going to be four foot long, and your station platform will be longer still.

You can just about have a 18” radius curve, but some stock will not run on it, and it is not advised. If you want double track, you are looking at a diameter of at least 4′, preferably rather more. This really requires a central operating well to ensure you can reach (i.e., you stand in the middle, with the trains going round you) if you want a continuous circuit, which means either ducking under or a lifting section.

OO has the biggest range of stock, and at exhibitions you are far more likely to find bargains in OO than other scales. OO is somewhat the benchmark against which I will compare the others.

The club’s layout Euxton Junction is OO.


N Gauge

Scale: 1/148 (about 2 mm to 1 foot)
Gauge: 9 mm

The second most popular scale is N, which is half the size, so your four coaches and a tank will be only two foot long.

You can have a 12” radius track with no problem, so a double track circuit is quite reasonable in 3’ width. This means you can have continuous running without needing to duck under or have a lifting section, which is a notable advantage.

It does not have quite the range of OO, and is I think slightly more expensive, but there is not much in it – but finding bargains at exhibitions is much harder, in my experience!

N gauge is excellent at “trains in the landscape”, where you model a railway that is part of a wider scene. It is perhaps not so good for shunting, as the standard couplings do not lend themselves to automatic uncoupling. The club’s layout Tellum Summit is N gauge, and shows the “trains in the landscape” well.


O Gauge

Scale: 1/43.5 (about 7 mm to 1 foot)
Gauge: 31.75 mm

The third most popular scale is O gauge, which is nearly twice the size of OO, so your four coaches and a tank engine will be seven foot long. Clearly if you are running passenger trains, you need a lot of space.

Most O gauge models are small shunting layouts, as the minimum radius is 7′, and ideally more than that.

O gauge has a much smaller range of models on offer, and they are significantly more expensive; however, the prices have come down a lot in the last few years, and where you might have paid over £1000 for a loco 5 years ago, it only be a few hundred now.

O gauge is excellent for small, super-detailed scenes. Most O gauge layouts are fairly small, and all about shunting. If you want a continuous circuit you need a lot of space.

Preston & District MRS has no O gauge layouts, but you might also want to talk to Preston O Gauge Group; they hold running days every month, and is worth a look just to see what they do.


TT Gauge (TT 3 mm)

Scale: 1/102 (about 3 mm to 1 foot)
Gauge: 12 mm

TT gauge is about half way between OO and N, and is nominally 3 mm to a foot. As with the other scales, British TT gauge had over-sized locos running on the same track that was used across the world. This scale is known as “TT 3 mm”, and was popular for a while in the fifties and sixties, but declined when the UK manufacturer, Tri-ang, went out of business.

It still has a following, and we have one member who is into TT gauge, but it is very niche. Expect to be scratch-building and fixing old locos.


TT Gauge (TT 120)

Scale: 1/120
Gauge: 12 mm

Just recently (2022) two manufacturers have announced plans for TT, but in the correct scale for the track size, 1/120. This is known as TT 120. One manufacturer, Heljan, subsequently dropped out, but Hornby has made some limited releases. Peco offer TT track.

In my view there is no guarantee Hornby will continue to support TT; if it takes off, they surely will, but if it does not, they will just as surely pull out. This makes it a somewhat risky long-term prospect if you are considering building a layout. There is a chicken-and-egg problem here. There needs to be enough models available to attract modellers to buy it, but until modellers are buying it, manufacturers will be reluctant to release models.

TT 120 is popular on the continent (and Peco are undoubtedly targeting that market too).



As all four of the above have a gauge that is slightly out of scale, there are alternatives that seek to rectify that. TT 120 has already been mentioned.

EM gauge (replacing OO gauge)
P4 (protofour) (replacing OO gauge)
2 mm finescale (replacing N gauge)
ScaleSeven (replacing O gauge)

I have no experience of any of these, but I believe all require the modeller to build his own track, and either build or modify everything to run on it.


Narrow Gauge

There are also various gauges associated with narrow gauge railways. As far as I know, the only one that has both track and ready-to-run stock is OO9.

OO9 is the same scale is OO gauge, but running on 9 mm track (the same as N gauge). There is a small,  selection of locos, wagons and coaches available RTR, all of which have appeared the last five years or so; they tend to be a little more expensive than OO or N gauge. The range seems to be growing at quite a rate.

There is a wide range of kits available, but in my experience getting a chassis for a loco is quite a problem.

OO9 lends itself to small spaces because the prototype railway adopted a narrow gauge specifically to handle small spaces. A tank engine pulling six coaches might only be a foot long by virtue of the fact that the real narrow gauge railways often used short, four-wheel coaches. Similarly a 9″ radius curve looks perfectly reasonably because the real railways had very tight curves.

While nowadays narrow gauge is often associated with the many heritage railways in Wales, narrow gauge railways were very common at one time on industrial sites from gas works to breweries to railway works serving standard gauge rolling stock.

There are a lot of other combinations of scale and gauge in the narrow gauge area; I am not going to list them all.


Other UK Gauges

Several other gauges are (or have been) available.

Gauges 1 to 5 are larger scales, defined in 1909, with 5 being the largest, and 1 the smallest (O gauge was so named as the next down from 1). Most have fallen out of use.

Gauge 3 is 1:22.6 scale, and is the smallest scale able to pull real passengers. This – and larger scales – is the realm of miniature railways, rather than model railways, and outside the remit of this article, but if you are interested you might like to contact Leyland Society of Model Engineers.

Gauge 1 was originally defined as 1:32 scale, but is now considered to be 38 inch to 1 foot, which is almost 1:32, but not quite. The main railway at Bekonscot model village is gauge 1 – the oldest model village in the world – and the earlier series of Thomas the Tank Engine used gauge 1 models (later they were made with CGI). There is a gauge 1 society.

G gauge is a popular choice for garden railways – if you have been to our exhibitions, you will have seen G-Whiz. The gauge is 45 mm, and a scale of 1:13 to 1:29, so is used for modelling both standard gauge and narrow gauge on the same track. It is close to gauge 1.

S gauge is exactly half gauge 1 (as originally defined), a scale of 1:64 (between O and OO). It dates back to before 1900. Again, no ready-to-run stock available, but there are kits. There is an S scale society in the UK.

Z gauge has a scale of 1:220, and a gauge of 6.5 mm, making it somewhat smaller than N gauge. I am not sure if anything is available in British outline (there is a fair bit of continental stock available). It is most notable in the UK for its use as a basis for narrow gauge trains in N gauge (called N-Z or Nn3 in the US).

T gauge is a third the size of N gauge, and first appeared in 2007. Only a couple of UK outline trains are available as far as I know, and to me it is little more than a gimmick.


International Standards

All the above are unique to the UK. Some common outside the UK include:

HO (pronounded “aitch-oh”): The most popular gauge in the US and Continental Europe, this is the same gauge as OO (16.5 mm), but the scale is 1/87 (3.5 mm to 1 foot). HO is so called because it is half O gauge.

N gauge: In the US and Continental Europe, this is the same gauge as the UK (9 mm), but the scale is 1/160. In Japan, N gauge is usually 1/150.