Layout Design - What will Fit

This article first appeared in the January/February 2004 issue of AMRA's 'Journal'.
By Stephen J Chapman.

Planning your layout is not the simplest of tasks. A large number of people when planning a layout imagine that they can fit much more railway into a given space than will actually fit there. The problem is that drawing a scale plan is a relatively time consuming and difficult task that many layout planners do not undertake. Even where a scale drawing is made, it is extremely easy to get something slightly out of correct scale and make it look like you can fit something in that just wont fit when you actually start construction.

There is a solution and it is quite a simple one. The idea is to divide your layout into "squares" each of which will hold a fixed and known quantity of railway trackage. Having determined the number and arrangement of the squares that make up your available area you can draw simple sketches of proposed layouts that you know will fit even though they are not drawn to scale.

How does this work? Well we set the size of the squares based on certain track configurations and work out the number of squares that will fit into our space. Since we will know exactly what track formations will fit into a square we will know that a given plan will fit our space if the number of squares required for the track configurations on the plan matches (or is less than) the number of squares in our given space.

So we should begin by measuring our available space. If this isn't a simple rectangular area then you may want to draw a sketch of the area and mark the critical dimensions on it. This is the maximum area available for our railway and from it we need to subtract access space. Passageways wide enough to walk along ought to be at least 500mm wide while you can get away with 300mm if the space is only there to enable access to otherwise unreachable tracks. We will also want to keep all track within 900mm of an accessway and all pointwork within 600mm (closer if the board will be higher than waist height).

The next step is to convert all of this information into squares. So the question that now needs answering is "how big is a square?" The answer to this is dependent on your minimum main line radius (r) and the distance you are using between track centres on double track (c). The length of each side of our squares is given by the formula 2c + r ie. the sides of our squares should be the minimum main line radius plus twice the distance between two parallel tracks. Note that this makes the calculations based on squares completely independent of the scale that you are modelling in, it works just as well in Z scale as in O scale even though the actual size of your squares will be different depending on your scale.

You can now recalculate your available space into squares by dividing each of the distances you have measured for the area by the size of the squares that you have determined. You should also work out how wide your passageways need to be in terms of squares. This gives you a final plan for the overall size of the layout as an arrangement of connected squares. Where you have fractions of squares you should round these down to the nearest half square when working out the final arrangement of squares.

You may also want to consider the maximum number of squares that can be fitted into your given space using the minimum possible radius for curves in your given scale or the maximum number of squares using the minimum radius that your rolling stock can actually operate through. This will give you an indication of how much extra you can fit into your given space if you reduce your curves to a minimum. The bigger the difference between this and your chosen radius, the more opportunity that you have to reconsider the size squares to use if your preferred track formation doesn't fit. Examples of minimum square sizes that you may want to consider are 450mm for HO scale and 300mm for N scale. If you are not going to be flexible about minimum radius curves then you don't need to consider this.

You might end up with a sketch like the following one where we have determined that we can fit seven squares across and five squares down to fill the available area. You need to work out what fraction of a square is needed for standard and minimum access spaces. You also need to consider how many squares you will be able to reach across and provide the necessary access to all areas of the layout. You can indicate your preferred allocation of access space by placing crosses through the full or half squares that you want to allocate to this. In the examplee we have set aside seven full squares and five half squares to allow access to the various parts of the layout.

Marking out squares in our available area

This gives us an arrangement of squares that we can use to draw sketches of track plans that we know will fit our available space because we know how much track can be fitted into a square.

How much track is that? Well let us work it out. Because of how we have defined the size of our squares in terms of our minimum radius and track centres we know that we can fit a quarter of a circle of double track into a square provided that the tracks are located towards the outer edge of the square. Realigning those tracks from the edge of the square to its centre also takes up a square and a crossover between the two tracks requires about two-thirds of a square.

Quarter turn, crossover, and curve to centre

Using this information it is quite simple to sketch in the mainline for our layout. The other thing that we need to know is the length that is required to split one or two tracks into a number of parallel tracks. It turns out that we can convert one track into five in the length of two squares using a simple ladder of turnouts. By adding a further ladder of turnouts on the other side of the first track we can expand one track into eight in the same length. Of course if we start with two tracks we can't expand on both sides so in two squares we can convert two tracks into ten. Even if you have defined your minimum radius to be the same as the minimum radius available in your chosen scale you should still be able to get at least ten tracks to fit across the width of one square so the number of tracks across a square will seldom need to be calculated.

One track becomes eight Two tracks become ten

Knowing how much track will fit into a square and how many squares we have for our railway we can easily sketch out a variety of track formations that we know will fit our available space. We can try out a number of different arrangements without having to draw any of them out to scale because we know that they will fit our space. This will help us to come up with the "best" trackplan that suits both our operating requirements and the available space without having to waste time drawing up a whole lot of unsuitable plans to scale. Of course there may be alternate ways that you can arrange access so if a proposed track plan requires tracks through those squares you have crossed out you may consider whether there is an alternate way of arranging access that will allow that formation to be used.

What about if our selected trackplan can't fit into the number of squares that are available regardless of how we rearrange the access? For example with the seven by five space diagram it is possible to get a continuous run into the avaliable area but only just. Well the size of the square is dependent on the minimum radius that we are going to use. Provided that you are not already at the smallest radius for your chosen scale, you can always consider the possibility of reducing the minimum radius that you use and hence make your squares slightly smaller. This will enable you to fit more squares into your given space and may enable you to get your track plan to fit. It is up to you how you compromise between track formations and minimum radius curves. Another alternative is to look for more useable space, again in our seven by five example we could get a continuous run in much more easily if we provide a lift up (or duck under) connection across the left hand end (where the leftmost two crosses are). In each case we are trading one aspect of our design (minimum radius or walk in access) for another (desired track formation).

Following this approach of determining the number and arrangement of squares that will fit our given space, we know that our chosen plan will fit the available space. No nore do you run the risk that a slight miscalculation on your scale drawing means that your selected track plan wont fit the available space. In fact, because we rounded down the number of squares there may actually be slightly more space available and you may find that when you actually come to build the layout that you may be able to squeeze in an extra siding.

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Copyright Stephen Chapman