In planning a layout, care must be taken to observe the size requirements of certain track formations. It may be possible to compromise on sizes where space is limited, but this will most likely be at some cost to visual effect, reliable running, train length and operational possibilities. Time should be taken to think through these concerns at the planning stage.

The recommended minimum radii for types of rolling stock are based on experience, but should be treated as guidelines, not rules. Tighter curves are possible but to achieve reliable running there must be some compromises with some or all of:

• Increased play in couplings and buffing gear,
• Sideplay in coupled wheels,
• Gauge widening,
• Increased clearances between adjacent tracks and between tracks and platforms.

If the area available for the layout permits, larger curves will not only make train movements that much easier but also add to the appearance of the track. This table also specifies, within limits, the type and general dimensions of the appropriate rolling stock.

Using this information, determine at the outset the minimum radius both for mainline curves and for yards and spurs. The running lines should be able to take the largest projected locomotive, carriage sidings the longest coaches without buffers locking, goods spurs shunting locos and an adequate number of wagons. Though generally this should mean mainline curves of 1830mm (6ft) radius, a careful builder in O fine standard can usually get even the longest loco round 1370mm (4ft 6in) and some even contrive to get big locos round 910mm (3ft) curves. For ready-to-run locomotives the manufacturer should be consulted for advise on minimum radii. Kit-built locomotives will have their own limits but at the building stage it may be possible to increase sideplay or bogie swing. Again, consult the manufacturer or study any reviews of the kit.

However, big locomotives on sharp curves do not look prototypical because of the curvature and the large overhang. Try to use the largest radii that can be accommodated and enter the sharp ones with a transition curve. Good use of transition curves will enhance both the running and appearance of the layout.

The lengths of trains it is intended to run are critical, particularly in the design of stations and storage yards. Prototype trains were sometimes but not always longer than is practical to run in model form on all but the largest layouts. The lengths of some typical trains are listed in Table 1 for guidance, and you should be able to use this information to determine the lengths of trains for your intended layout. Alternatively, the size of the layout may be used to decide on the length of trains you can run and you can use this table to turn that length into a number of coaches or wagons.

Table 1. Some typical model train lengths.

At a terminus station where a run-round loop is provided, it must accommodate the longest train that will operate at that station. This is the clearance length shown in Figure 1. To this must be added the fouling distance which allows the locomotive that is running round to clear the train, and at the end of the loop, the track extension beyond the point must be big enough to take the longest locomotive.

The overall length is equal to 2 x Loco length + 2 x Fouling distance + total length of the rolling stock.


Figure 1. Run round loop length.

Results for the trains listed in Table 1 (except the multiple unit for which this does not apply) are listed in Table 2 as a guide. At larger stations there may be a separate loop that is part of the goods yard where goods trains can be run round, but the same dimensions will apply here. For an overtaking loop on the main line where a fast train can overtake a slow one, the clearance length (Figure 1) only applies.

Table 2. Typical run-round loop lengths based on the train lengths from Table 1.

When a layout has to provide for continuous running then space must be available, at the very least, to accommodate a simple circle. One or more passing loops will be most desirable, especially if the layout belongs to a group. A double oval with connecting crossovers and several loops on each circuit should be provided where the group meets regularly and has several members.

The space needed for any layout of this type can readily be assessed by dividing the layout plan into squares. This approach was introduced by the American layout designer John Armstrong. The method can also be used to see how much can be got into a given space. Briefly, a square is the space occupied by a 90° double-track curve where the centreline of the inner track is the minimum radius employed on the layout. The length of each side of the square is equal to the minimum radius plus twice the distance between track centres (Figure 3). Some typical square sizes, based on the recommended minimum radii, are shown in Table 3. The distances between track centres of the sharper radii have been increased to allow for vehicle overhang on curves.


Figure 2. John Armstrong's layout squares.

The use of these squares is not confined to 90° bends but can also be used for partial bends and for planning reverse curves. Space for transition curves can be introduced by separating and offsetting the squares.

Table 3. Typical square sizes.

The actual size of oval suitable for continuous running depends on a number of factors; the length of trains; the length occupied by the station area if it is accommodated on one of the straight sides; etc. A reasonable estimate can be made, using the information given above, to calculate the minimum space required to accommodate typical trains. From that estimate it should be apparent whether the layout can be fitted into the space available.

The space estimate is two squares wide by two squares plus the run-round distance long. Shortening the run-round length by having the loco run on to the curve, etc., can help squeeze the overall length but could make coupling and uncoupling more difficult, particularly with the smaller radii. Table 4 gives some guide sizes using the squares in Table 3.

Table 4. Minimum oval sizes.

This article was originally written by the Technical Committee for the Gauge O Guild Manual. It was edited and adapted for the GOGWiki by Nick Baines.