Handy Guide to Scales and Gauges
Confused by the different sizes of railway models available? If you're thinking about taking up railway modelling, and aren't sure yet exactly what's meant by "OO", "O", "N", etc? Here's a handy(ish) beginer's guide to the most common scales and gauges used in Britain.
Just before getting into the lists, it's important to clear up the difference between "scale" and "gauge". Scale is the ratio between the original and the model. So, for example, a model that is one-tenth the size of the original would have a scale of 1:10. A scale can also be expressed in terms of conversion units; so for example OO uses a scale of 4mm to 1 foot - that is, one imperial foot of length in real life is represented by 4mm on the model. Before you ask, yes, this mixing of metric and imperial is rather inconsistent but there is a good reason for it. Trust me.
Gauge, on the other hand, is the width of the track - that is, the distance between the rails. If you're modelling "ordinary" trains, the ones you'd use on a nomal rail journey in the UK (and most of the rest of the world), then the gauge of the model is obviously directly related to the scale - the smaller the scale, the narrower the gauge. But not all trains run on standard gauge track, and modelling narrow gauge trains (such as the Welsh narrow gauge railways) is also very popular. Also, the exact distance of the gauge for any particular scale may well be a rather awkward value that's hard to measure, so most common gaauges round it off to a greater or lesser extent. But some people are less happy with approximations than others, so you'll often find that there's more than one gauge for any common scale.
Anyway, enough of that. On with the lists. Starting with gauges and scales used for models of standard-gauge originals...
- 1 Gauge. (Scale ratio 1:32, track width 45mm) This is the largest gauge for which ready-to-run (RTR) models are generally available, although they're very expensive - this isn't a gauge for someone who wants a model railway on a budget! 1 Gauge is commonly used for garden railways, as it's a bit too big for indoor use unless you have a very large house. It's also the point at which railway modelling blurs into model engineering or miniature railways - the "number" gauges start at 1 and go all the way up to 6 Gauge, but at the upper end of the list you're talking about the miniature railways often found in parks.
- O Gauge. (Scale ratio 1:43.5, 1:45 or 1:48, track width 32mm). This is the largest gauge commonly used for indoor modelling, and has the advantage that it's large enough for outdoor use as well. It's called "O" because it was originally intended to be the lowest of the number range (ie, the next down from 1), but these days it's invariably pronounced "oh" and spelled with a capital letter O rather than being called "zero". O was the original "toy train" gauge, and still has a lot of toy-oriented products available. The plastic "Big Big Trains" that a lot of us grew up with are O Gauge, and the model railway which appears in the Wallace and Gromit feature "The Wrong Trousers" appears (as far as you can tell in an animation!) to be O Gauge. For the modeller, as opposed to the toy enthusiast, O Gauge gives a lot of opportunity for detail work and layouts in this gauge are always among the most impressive at any exhibition. However, a large O Gauge layout needs a lot of space, and you'll need to be pretty dedicated to making your own models as there isn't a lot of RTR stock available.
- S Gauge. (Scale ratio 1:64, track width 22.42mm). Half the size of 1 Gauge and a bit smaller than O, this certainly isn't for beginners as there's virtually nothing available that you won't need to build yourself. But it has a dedicated (though rather small) community of modellers as it provides a good compromise between the opportunity for detail offered by O gauge while not needing as much space.
- HO Gauge. (Scale ratio 1:87, track width 16.5mm). Called "HO" (and always pronounced "H-O", not "ho") because it's half the size of O, this is the most commonly used scale pretty much everywhere in the world except the UK. If you want to create a model layout based on an American or European original with the minnimal of effort, then this is easily the best choice for the beginner. You can create entire layouts using entirely RTR rolling stock and there's plenty of scenery and building models available as well. However, there are virtually no RTR models of British trains available in HO. Why not? Well, read on...
- OO Gauge. (Scale ratio 1:76.2, track width 16.5mm). This is the most popular gauge in the UK, by a large margin. (It's pronounced "O-O", not "oooh"). Like HO elsewhere, it's ideal for the beginner as you can create a model railway using only commercially available products. But it's a lot less than ideal for the more pedantic modeller, due to a major inaccuracy in the specification which came about as the result of a very early design compromise. When HO was introduced (in the 1920s), it was right at the cutting edge of model miniaturisation - you simply couldn't go any smaller and still have a working propulsion mechanism (either clockwork or electric) inside a model locomotive. But real life British trains are slightly smaller than those elsewhere, so when they were modelled in HO the mechanism didn't fit. As a compromise, models of British trains were made slightly larger, but still used the same track gauge as HO, so that they could use the same internal mechanisms as HO models. This slightly larger scale was called OO. However, this means that OO trains run on track that is slightly too narrow for the scale - to be accurate for the scale ration, it should be 18.83mm instead of 16.5mm. A difference of just over 2mm may not seem a lot, and for toy trains (the original market of OO manufacturers) it's perfectly acceptable. But, if you want your models to be accurate to fractions of a millimetre, then it's far too large a gap to overlook. So, while OO is good for the beginner and fine for the typical modeller who isn't too concerned about that level of accuracy, there are other gauges using the same scale ratio as OO which are more accurate at track level.
- EM Gauge. (Scale ratio 1:76.2, track width 18.2mm). Like OO, uses a scale ratio of 1:76.2 (or 4mm to the foot), but has a more accurate track width. Not for the beginner, as there are no RTR models available, but for experienced OO modellers looking to make the step to finescale modelling it's a good option as the ancillary equipment (such as lineside kits, scenery, etc) is the same and the techniques are similar.
- P4 Scale. (Scale ratio 1:76.2, track width 18.83mm). P4 is a more accurate (and more recent) approach to solving the same problem as EM. As with EM, it isn't for the beginner, but it has the same benefits as EM and is now the leading finescale gauge using the "4mm" ratio of 1:76.2 and has a thriving modelling community.
- TT Scale. (Scale ratio 1:120 or 1:101.1, track width 12mm or 14.2mm). Slightly confusingly, TT can mean two different scales and two different gauges (although the larger combination of 1:101.1 on 14.2mm track is often known as "3mm Scale" as it has a conversion ratio of 3mm to the foot). TT was an attempt by manufacturers to make models smaller than HO/OO; it's now no longer commercially available but has a small community of modellers. Not really suitable for a beginner due to the lack of available products.
- N Gauge. (Scale ratio 1:148, 1:150 or 1:160, track width 9mm). The N" stands for "nine", being the width of the track in millimetres. As with OO/HO and TT, British models are slightly larger than those made elsewhere but use the same track and mechanisms, thus resulting in a slightly too-narrow track for British versions. N is the second most popular scale/gauge worldwide (after HO/OO, and before the third most common, O), although RTR products in the UK are only generally available from one manufacturer. N's main advantage over larger scales is its ability to fit a lot of railway into a reasonably small space, making it ideal for people who like to create authentically-proportioned models of actual track layouts and run authentically-proprtioned trains (in most larger scales, track layouts usually have to be compressed to fit the available space). The main disadvantage (other than the relative shortage of RTR products) is that it's too small for detailed modelling (unless you like working using a microscope) so it has less appeal to those who like working at individual unit level. Provided you are happy with these drawbacks, though, N can be a good choice for a beginner, particularly if space is at a premium.
- Z Gauge. (Scale ratio 1:220, track width 6.5mm). Until recently this was the smallest commercially available scale. Originally introduced by German manufacturer Marklin, there is now a reasonable range of products depicting originals from Europe and the USA, but none (so far) of British prototypes. At this size, though, it's closer to an executive toy than a detailed model.
- ZZ Gauge. Scale ratio 1:300, track width 4.8mm). Currently only available in Japan, and only of a very restricted range of products, I mention it here purely for curiousity value!
For models of narrow gauge originals, there are also several popular gauges:
- G Gauge. (Track width 45mm). Uses the same track width as 1 Gauge, but models use a larger scale to represent narrow gauge trains. Most commonly used in garden railways.
SM32. (Track width 32mm). Uses the same track width as O Gauge, but uses larger scales to represent narrow gauge trains.
O-16.5. (Scale ratio 1:43.5, track width 16.5mm). Uses the same track width as OO/HO gauge, but with a scale ratio the same as O Gauge.
- OOn3 Gauge (Scale ratio 1:76, track width 12mm). Uses TT-sized track with 4mm scale models (the same scale as OO) to represent the Isle of Man or Irish 3ft railways.
- HOm Gauge. (Scale ratio 1:87, track width 12mm). Uses TT-sized track with 3.5mm scale models (the same as HO) to represent the Swiss metre gauge railways.
- OO9. (Scale ratio 1:76, track width 9mm). Uses N-sized track with 4mm scale models (the same scale as OO). This is probably the most common narrow gauge scale/gauge combination in the UK and can be used to represent a typical Welsh narrow gauge railway, industrial railway or quarry line. OO9 models are often used to provide secondary interest on an OO layout, or used to create working dioramas in restricted space.
