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The track layout is not important, though arranging the test track so you can do some shunting or even run round a short train will add to what you can learn from it. Some test tracks end up with scenery and become more like a proper layout, but really it is best to regard the test track as a proving ground for building and laying track and getting something running. Then, using that experience, you can design and build a proper layout.
With a better idea of what's involved and the time and skill needed, your thoughts can turn to a more complete layout project. Often, this is where the difficulties really start - there are just too many choices and too much to think about at once. Choosing a period, railway company, type of railway operation, geographical location and so on is one part of this. Designing a track layout that will fit the space you have and have some chance of being a believable representation of your chosen prototype is when harsh reality closes in. The fact is, however much space you have, your imagination can always ensure it is not enough! But see overleaf.
This short section cannot attempt to deal with all the issues involved in designing a layout, and so we concentrate on the particular issues that arise with modelling to P4 standards.
It helps to have a clear idea of the rolling stock you expect to run on the layout as the choice of curve radius and turnout size may limit what you can run. There are no absolute rules as all sorts of tweaks can be applied to help large locomotives round tight curves, although sometimes at the expense of realism.
In fact, how it looks is often the best guide to what it is worth attempting to do. For example, a dockyard or industrial layout with small shunting locomotives and short wheelbase 4-wheel wagons can look good with curves of 600 - 800mm radius and use A5 turnouts - but bogie wagons, coaches and large locomotives, even if they can be persuaded to run reliably round such curves, will never look right.
You will find many different views on this, but it may be helpful to give some rough guidelines to work to: For large steam locomotives and main line bogie coaches, aim for a minimum radius of 1200mm (4' 0"). For smaller (eg: pre-Grouping) locomotives and coaches, a minimum radius of 1000mm may be acceptable.
Diesel and electric locomotives do not suffer as much from excessive overhangs and long fixed wheelbases as large steam locomotives, so a minimum radius of around 1000mm may be acceptable for even the largest modern rolling stock - but use transition curves and take care with reverse curves (eg in crossovers) to avoid problems with vehicle end throws.
If rolling stock is restricted in wheelbase and overall length, much tighter curves can be used, but care must still be taken to avoid 'buffer locking' caused by excessive end throws.
For running lines, use turnouts with B or C switches. If possible, aim to use nothing tighter than a B7.
In yards and sidings, turnouts with A switches save space.
Some of this may seem very restrictive if you have until now worked with ready to run rolling stock. There is no doubt that moving to P4 involves accepting that less railway will fit in a given space than if, for example, OO gauge is used. What may be surprising is what can be fitted into a modest space and this was well illustrated by the Scalefour Society's '18.83 Challenge' - when a large number of very different exhibition layouts were produced - some by complete beginners in P4 - each occupying an area no greater than 18.83 square feet.
The top picture not only shows the runner-up in the '1883 Challenge' but also the winner of the photography competition at the event. Simon Challis has convincingly captured the Somerset & Dorset Joint at Cheddar in an area of only 18.83 square feet.
With the help of Colin Brown, Alastair Dickson set about recreating his home territory as he remembered it in the 1960s, with Colinton, above right. A simple plan, bottom, it offers much of interest, and again met the '1883 Challenge'.
If you want to go narrow gauge, or broad gauge, P4 can replicate them too, as Tony Miles' Adavoyle shows.