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When we sit down with pencil and paper to sketch our layout ideas we tend to try and put a complete railroad empire onto an 8 x 4 foot board. This always looks great on paper, but when we come to build that dream layout we are often bitterly disappointed.

The often heard cry is `but it fitted on the drawing! How can we stop this anguish and make our doodles reflect reality and give us a fairly accurate idea of what will fit into our available space. This is especially critical if we have only a small space available to us.

With this in mind, there is a way that we can make our doodles give us a more accurate view of what will fit into a given space. These are called `LAYOUT DESIGN UNITS’ or UNITS for short. A UNIT is simply the average length of the rolling stock to be used on the proposed layout. If for example you model the Rio Grande Southern, which used standard 30 foot cars, one UNIT equals 30 feet or in O scale, 8 inches. Everything that is going to be included in our layout doodles can be expressed in these UNITS. Using our Rio Grande Southern example again, a locomotive is approximately 12 inches long or 1.5 UNITS long. A depot building might be 16 inches long by 8 inches deep, which is 2 UNITS by 1 UNIT. Likewise, a No.5 turnout in On3 is 12 inches or 1.5 UNITS long.


When I sit down to put a layout idea on paper I prefer to use graph paper with a 1/4 inch grid, but any graph paper will do nicely. I can then call each grid square 1 UNIT or 2 UNITS etc. depending on how big I want to draw my layout idea.

With a sharpened pencil, graph paper and plenty of optimism, we can settle into our favourite armchair and get down to the serious business of putting our layout ideas onto paper. Important items first - get a cup of freshly brewed coffee or a can of your favourite beverage and settle in.

The first thing I do when starting a doodle is to note the objective i.e. what I am trying to achieve? This is handy if you come back to the drawing later on. I also date the drawing for the same reason. I then list the main objects I want to include in the drawing, along with their UNIT size. 

We might be putting our ideas on paper for various reasons. We could be developing a layout idea from scratch, trying out an idea for an existing area on our layout, designing a new area of our layout or just seeing what will fit into a known area that we have available.




Debate has long raged about the design and what is the minimum space required to build a switching module which has reasonable operating potential. The size of this module will vary greatly with the scale and the size of the prototype being modelled. This is where ‘Layout Design Units’ show their real potential.

  • Scale: On3 (1/4” to the foot)

  • Locomotives to be used: Bachmann 8 ton Porters : 15 scale feet long

  • Standard Car length : 20 scale feet

  • Turnout Size: No.4 Blade type : 30 scale feet long

  • Number of cars in a train: 3 (no caboose will be used in this example)

  • Number of switching locations: 3

With this example, the standard length of a piece of rolling stock is 20 feet or 5 inches in O scale. This gives us the first dimension for our drawing i.e. 1 UNIT equals 5 inches. We can now work out that the Porter locomotive is 15 feet long which is 0.75 UNIT long and a No.4 turnout requires 1.5 UNITS .

To help us get started we can use some of the basic techniques used in designing switching layouts:

  • Minimum length head shunts

  • Minimum number of spurs

  • Reverse direction spurs

  • Switching locations on head shunts

  • Multiply switching locations on spurs


Finally we can get down to relieving our pent up frustration's and get our ideas onto the blank sheet of graph paper sitting in front of us. Remember we are only doing rough sketches at this stage to get started and to see if our ideas are realistic.

The initial concept for this switching module could be something like this -

A mainline and passing loop at the front to hold a 3 car train, with a spur to an industry, and a reverse spur off that one, where two industries are located. The switching plan is to move each of the cars to one of the three locations on the spurs in a random order and then to make up a three car train back on the main line in a predetermined order. More about this latter.

As we begin sketching we have no idea how big this module is going to be. Lets start with the main line, which we will place at the front of the module, as this will give us an idea how long the module will be. The passing loop must hold three cars with clearance at the turnout. So lets mark off three UNITS for this. Now we mark off 1.5 UNITS for the turnouts at each end, and after that, the head shunt will have to hold the locomotive and one car, which is 0.75 UNIT for the locomotive and 1 UNIT for the car. We might be a bit generous and allow an extra 0.25 of a UNIT on the head shunt to give us some operating space. If we allowed 1 or more UNITS here we could shunt two cars at once, but that would be make the shunting too easy. We can now estimate the length of the main line, in UNITS, which would be a total of 10, multiplied by 5”, will give a required minimum length of 50 inches, or just over 4 feet .

If we were modelling this same track plan for the Rio Grande Southern in On3 (the same scale and gauge as this example), the length would considerably longer. The standard length of a piece of rolling stock is 30 feet (as compared to 20 feet) so 1 UNIT which equals 8 inches as against 5 inches in this example. A locomotive and turnout are both 12 inches or 1.5 UNITS long. The length required would be 11.5 UNITS , but in this example 1 UNIT equals 8 inches, thus the required space is 11.5 UNITS by 8 inches, which equals 92 inches long - almost twice as long as the first example.

Back to the first example. We can now add the first spur. From a half way point on the turnout, draw a line at an angle of 1 in 4. This line can continue till it is in line with the end of the main line. The end of this spur will be the head shunt for the second spur which runs in the reverse direction

Now that we have our sketch finished we can estimate the depth of this module by counting the squares up, which would be 2.4 UNITS or 12 inches deep. Its hard to believe, but this minimum switching layout in ‘O’ scale will fit into about 4 feet by 1 foot! In Ho scale it would fit into about 3 feet by 9 inches.

There are many ways this layout can be changed to suit different requirements. It can be mirror imaged, reversed, stretched in any direction, or a switch back can be added to give more interest. Sidings could be added to the other side of the mainline. This example can be the basis behind many layout designs of any size or scale.



While this small module may not seem to be very challenging as a switching layout, it is surprising what can be done. Let’s consider a very simple system. We can place a switching location on the end of the first spur, and call it ‘Location A’, and then place two locations on the reverse spur called ‘B’ and ‘C’ (see diagrams). Not knowing how many cars will make the switching interesting, let’s try a train of a locomotive and 3 cars on the main line.

Let’s make each car different - say a box car, a gondola and a flat car.

Lets keep it simple to start with and say that any of the three cars can go to any of the three switching locations. Now we need a system to tell us where each car has to go. A simple board with the 3 locations marked on it and an area for the train on the mainline should do.

All we need now is a card for each car (see opposite).

With the train sitting on the mainline, shuffle the cards and place 1 card at each location on the board. Then you have to move the cars to those locations. Once this is done, the cards can be picked up, shuffled and placed on the board. You then have to reassemble the train in that order.

It could be made more interesting by adding a forth car to the yard, say a tank car, The train to be switched or made up would still be only three cars, but now a random factor has been added. Plus the extra car would make it that much more awkward to switch the module.

This was the way that my layout, "Old Ophir" was designed, and it has proved to be a very challenging and interesting layout. To see this article, click the link above.