I had rather assumed that the normal "MOMING" correspondent would have mentionned this by now but as he hasn't perhaps I can draw your attention to an interesting challenge which appears in edition no.70 (july-September 2012) of the French narrow gauge modelling magazine "Voie Libre" which I know is quite widely read in the UK.
The translation from the original French below, and the added notes, are mine. An official English translation can be found by going to the Voie Libre website (www.voielibre.com) and clicking on "English translation" for edition 70 - the challenge is detailed on page 26 of the magazine.
LR Presse, Voie Libre, MOMING and the Sedan RAMMA  are launching a new challenge - EuroMOMING 2013.
The challenge is simple - build a working ultra-narrow gauge layout within the confined area defined by the rules:
i. The layout must work and include at least one point.
ii. It must be the reproduction of a real  railway with a prototype gauge narrower than 60 cm, for example 15 or 18 inches or 40 cm .
iii. The maximum surface area of the layout relates to the model gauge chosen :
- equal to three A4 sheets (ie 21 cm x 29.7 cm x 3) for 9 mm gauge track
- equal to three A3 sheets (ie 29.7 cm x 42 cm x 3) for 16.5 mm gauge track
- equal to three A2 sheets (ie 42 cm x 59.4 cm x 3) for 32 mm gauge track
- the fiddleyard(s) are in addition to this but their size must remain reasonable.
iv. The theme and period are entirely up to you!
LR Presse has made its Voie Libre Forum available to describe your projects, discuss, create, exchange, etc. forum.e-train.fr (or select FORUM on the VoieLibre site).
PRE-REGISTRATION must be made with RAMMA before the end of December 2012 (www.ramma.org).
At RAMMA in October 2013, those who have taken up the challenge will be able exhibit the layouts that they have created; the best layouts will be reviewed in Voie Libre and other LR Presse publications.
My added notes:
 The bi-ennial RAMMA is my local show, despite being in another country and an hour's drive away. Although it also includes models other than trains, last year two large halls (out of 6) were devoted to an impressive range of layouts including a lot of narrow gauge and British layouts from Christopher Payne, Gordon and Maggie Gravett and John de Fraysinnet. Although it is in a sparsely (by UK standards) populated area and is only open Saturday afternoon and Sunday, last year's attendance figures would have had most UK club treasurers rubbing their hands in glee.
For anyone from the UK thinking of making a weekend of it (it is about a 3 hours drive from Calais) I can strongly recommend the Hotel Panorama in nearby Bouillon (over the hill in Belgium) - not many hotels are so welcoming that one feels to urge to move to the area, this one was and I did.
 The original French says "réel" which means real or prototype, however this must be interpreted fairly liberally as it would be nearly impossible to find a prototype ultra-narrow gauge railway situation that could be modelled accurately within the defined constrained layout size.
 Interestingly 50 cm, which was not only a very common French industrial gauge but is also the gauge of two important French tourist lines (one ex-SNCF), isn't mentioned but clearly meets the less than 60 cm rule. 40 cm, where it occurred in France, tended to be hand-worked.
 I assume that other gauges would be required to fit the same area as the nearest nominated one - thus for example, 6.5, 10.5 or 12 mm would equate to 9 mm and 14 or 18.2 mm to 16.5 mm. It would seem that within the rules one could have, for example, three mini-dioramas linked by blacked-out tracks, an excellent way, perhaps, of depicting outdoor sections of an 18 in gauge works railway - of which their were a number of examples in the UK.
In 7 mm scale the obvious prime track gauge contenders are 9mm for 40cm or 15in prototypes, 10.5 cm for 18 ins or 12 mm for 50 cm.
Otherwise, for those seeking a change to 7 mm:
in the 1:35/1:32 scale range 10.5 cm or 12 cm for 40 cm or 15 in, 14 mm for 50 cm or 18 in or even 16.5 cm for 50 cm,
in the 1:24/G scale range 16.5 cm for 40 cm or 15 in
and in 1:12 32 mm for 40 cm or 15 in
One final possibility might be 21 mm:1 ft scale on 32 mm track to represent 18 in gauge. Almost 40 years ago I used this very combination to create a model of the Horwich Works tank "Wren" and its tender which was battery powered and included working valve gear, reversing lever and regulator.