This article first appeared in the May/June 2003 issue of AMRA's 'Journal'.
By Stephen J Chapman

One recurring discussion amongst 4mm/ft modellers (to use that scale as an example) is the relative merits of 16.5mm gauge (OO) versus 18.2mm gauge (EM) versus 18.83mm gauge (P4). It is not just limited to that scale either, similar discussions take place in most of the other scales too (eg. O vs S7 or N vs 2mm). No matter what scale that you model different people have different ideas as to the accuracy that they wish to model to as compared to the range, convenience, and price of the relevant models and materials.

Fortunately, most of these discussions end with at least most agreeing that so long as we all have fun with the hobby that it doesn't matter what track gauge that you use.This is a sensible conclusion because for most of us no matter how inaccurate that the track gauge that we use is compared to the correct scale it is still just about the most accurate scaled dimension that we have on our model railway. Most of us just don't have the the space to model to anywhere near scale proportions when it comes to such things as distances between stations or even minimum radius curves. The compression that we use with squeezing our desired track plan into the available space makes the difference between the track gauge that we use and the correct scale track gauge appear so close to zero in comparison that the discussions of the relative merits of the different gauges appear to me to have little merit.

Before I start discussing scale distances let me first make it clear that I see nothing wrong with the selective compression that we all use on our model railways. This discussion of true scale distances is just so that we can consider just how vast such distances really are so that we can put into proper perspective the various compromises that we make to true scale when building our model railway.


Let's begin by considering just how much track is required to achieve a scale distance of one kilometre or one mile. Here are approximate figures for some of the more common scales.

Scaleone kilometreone mile
1:22 (G)45.5 metres (151 feet)72.8 metres (242 feet)
1:43.5 (O)23 metres (76 feet)37 metres (123 feet)
1:76.2 (OO)13 metres (44 feet)20.8 metres (70 feet)
1:87.1 (HO)11.5 metres (38 feet)18.4 metres (61 feet)
1:148 (N British)6.8 metres (22.5 feet)10.9 metres (36 feet)
1:160 (N)6.2 metres (20 feet)10 metres (33 feet)

Now on most railway lines even the closest railway stations are several kilometres (or miles) apart. Try multiplying the distance between two stations on your favourite railway line by the appropriate scale distance (from the above table) for your favourite scale and see just how much track you should really put between those stations if you are intending to model them to scale. Even if you intend to build an N scale model of two stations that are only two miles apart you still ought to include twenty metres of track between the stations if you are going to model the intervening distance to the correct scale. Even the modeller with the most space available is unlikely to allow more than four or five metres of track between two stations on their N scale layout giving a compression ratio of at least 75 percent and probably a lot more. In the larger scales you are even less likely to have the space available and would probably use an even greater compression ratio. Also, although many stations are actually much further apart than just a couple of kilometres, we are unlikely to allow more space between our stations to represent this greater distance and so the compression ratio ends up being even higher.

Another area where we use a rather dramatic compression is with the curves that we use on our model railways. Even in a dockyard where space is exceedingly tight you would be unlikely to find curves any smaller than say 100 metres radius. Such curves on the rare occasions that they occur would be traversed by trains travelling exceedingly slowly. Such curves would probably also have a check rail throughout to help stop the trains from falling off. A more reasonable minimum radius for branch line running would probably be about five times this (or more) and for a main line even larger radius curves would be the smallest that you would most likely find.

Let's consider this smallest radius from our dockyard though and see what it scales down to.

1:22 (G)4.5 metres (180 inches)
1:43.5 (O)2.3 metres (91 inches)
1:76.2 (OO)1.3 metres (53 inches)
1:87.1 (HO)1.15 metres (45 inches)
1:148 (N British)680 millimetres (27 inches)
1:160 (N)620 millimetres (24 inches)

As you can see, even this absolute minimum radius fully checkrailed curve exceeds even the most generous radius curves that we are likely to use on our model railways. Realistic radius curves for our branch line or mainline are well beyond what we can fit into even the largest available space. True scale curves can most easily be modelled using straight track since the variance over the length of a single piece of track is so small.


Many modellers avoid one or both of these problems completely rather than rely on selective compression. By modelling only one station and using a fiddle yard to represent the rest of the railway we avoid needing to compress the distance between our stations. By modelling an end to end layout along one wall we eliminate the need for curves on our layout. Doing both of these gives us the terminus to fiddle yard layout that is so popular with some modellers.

Just how far you are prepared to compromise your need for scale accuracy in constructing your own layout will depend to a large extent on where your interest lies. If you are more interested in constructing models then you will be less likely to compromise since your priority will be on the accurate appearance of the model, if you are more interested in operating the models then you will be more likely to compromise since you can increase operating potential by squeezing in more tracks. This isn't an either/or situation as most modellers have at least some interest in both constructing and operating. Each modeller will consider their relative interest in these two aspects in deciding how far to compromise on true scale and how far they are prepared to compress the distances in their layout.


Finally, let's consider one area where the degree to which we can apply compression is limited. This is the situation where we have one track that needs to go up and over another track. The distance required to climb to a sufficient height to pass over the top of another train will be determined by the gradient that we are prepared to use and by whether one line or both will be on a grade. Where we intend to build an entire section of our model railway at the higher level a greater clearance will be required to allow for structural supports under the higher tracks. As the depth of our support does not change with the scale that we use the smaller scales will require a relatively longer distance to provide the necessary clearance.

So how much distance do we need to take one track up to a height sufficient to pass over another track. One thing that simplifies our chart is that the smaller relative height of the British prototype means that the distances to give the required clearances work out to about the same as for the larger relative height of other prototypes built to the slightly smaller scale used by modellers of those prototypes. I will also leave the larger scales out of this chart since few of us are likely to have the necessary space to go up and over in those scales. First let's consider when we only use a gradient on one track (either minimum clearance or with full supporting framework).

Scale1 in 201 in 30in 401 in 50in 601 in 701 in 80
OO/HO min1520mm2280mm3050mm3810mm4570mm5330mm6100mm
OO/HO full2030mm3050mm4060mm5080mm6100mm7110mm8130mm
N min760mm1140mm1520mm1900mm2280mm2660mm3050mm
N full1520mm2280mm3050mm3810mm4570mm5330mm6100mm

As you can see, the distances required are quite large even with the steepest gradients and that is just the distance required to get the necessary track separation. Once we have gone over the other track we still require the same distance again to bring us back down to our original track level. There are only have two alternatives when it comes to trying to reduce the distance required in this instance. We can use steeper less realistic gradients to squash the climb into a shorter distance (although going much below 1 in 20 will result in many model locomotives not having sufficient power to climb the grade. The second option is to place both lines on a grade with one climbing and the other falling until we get our clearance. In this second instance the distances required will be exactly half of what we previously determined (assuming that we use the same grade on both tracks) which saves my having to include another chart for this.

The actual gradients used on the prototype railways varies from one country to another. The smaller (and hence less powerful) locomotives used in England meant that even one in eighty was considered to be a really steep grade where in Australia and the USA steeper grades than that are more common. This means that you have to comress the gradient more on a layout based on an English prototype to fit the grade into the same space than you would need to do for a different prototype. Again how much you are prepared to compress your grades depends on the balance between your modelling and operational interests. In fact as many model locomotives are effectively more powerful than the full sized article, you may deliberately make your grades steeper than the prototype in order to provide a reason for providing a banking locomotive or for double heading.

Another option to consider is to use different grades for the track up the gradient to that used for the track going down. It is much easier for a train to go down a steep grade tha it is to go up so provided that you can separate the two lines you may be able to shorten the overall distance by using a much steeper grade for descending trains than you use for ascending ones.

The opposite alternative is to put your entire layout on a grade so as to reduce the gradients to the absolute minimum. One portable layout that I started building (which never got finished once I decided that I didn't like portable layouts) had almost the entire mainline (which was about 24 metres long) on a less than one in two hundred grade which was just enough to go up and over with minimum clearances in N scale. This allowed me to have a twice around inverted figure eight layout without the gradient being all that obvious and without it having any impact on the operation of the layout apart from enabling me to have twice as long a run as I would otherwise have been able to.


Whatever you decide when it comes to selective compression of distances on your layout is up to you. You have to decide how far to compress what in order to most closely achieve your own ideals of what a layout should be. In most cases some form of compression will be required in order to make your layout fit the available space.

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Copyright Stephen Chapman