Greetings from the wilds of Southern California. I have had the great misfortune of being bitten by both the 4mm GWR bug and its very distant cousin, the American On30 bug. During my service in the US Army, I made many trips between Germany and the UK and in the process collected a considerable amount of OO scale GWR locos and rolling stock, as well as a lot of scenery items, in the hope of one day building a nice "little" West Country branch line layout. Fast forward to the present day, when now I finally have the time and space to start building a home for my pannier tanks, B sets, etc. Alas I have also fallen in love with Bachmann's amazing range of On30 scale locomotives and rolling stock. Who can resist the whirring motion of the geared Shays and Climaxes with their attendant sounds? I couldn't and now my train shed has five of these little beasts and enough rolling stock to support them. My dilemma, of course, is that I only have enough space to build one layout, and I want to keep and enjoy both collections.

After much thought, and given the happy fact that both scales run on the same gauge of track, I've decided to build a layout that, with some careful scenic planning, could accommodate both OO and On30 scales, although not simultaneously. My main challenge is to create scenery through which GWR branch line trains and US back-country logging and mining trains could both plausibly run. My thinking is to use a minimal number of buildings and structures in each scale that can be fairly easily be removed from the layout and stored when I wanted to "switch" scales.

Per the attached plan, I have a 10'x12' garden shed and plan to build this as a shelf layout. "Exford", the OO scale GWR version, is a real town on Exmoor that never had a railway in reality. My fictional history has a nominally independent company build a route across Exmoor linking the Somerset and Devon line from just west of Dulverton with the Minehead branch. The proprietors wanted to exploit the mineral resources on Exmoor as well as promote the scenic beauty of the area with a direct rail link. This hypothetical line has its main crossing place at Exford, which also serves as a junction with a light railway built to serve the village of Simonsbath with its many iron and copper mines. In reality, the mineral deposits on Exmoor played out very quickly but in this fictional world they continued to be marginally productive and so this backwater branch line managed to stay afloat. In my plan, the Simonsbath branch leaves Exford and, crossing the River Exe, continues on a 1 in 33 grade to the small station at Simonsbath. The "main" line to Minehead continues around the shed, crossing the river and running through countryside before eventually disappearing into a tunnel under the Simonsbath terminus.

I resurrected an old sector plate fiddle yard which just happens to be the same length as my shed door is wide so I plan to use that as a link between the Exford station area and the other end of the tunnel, using the straight track of the plate as a through route. I will install a loop inside the tunnel to hold longer trains that the sector plate can't accomodate. On the sector plate I can hold a 45xx Prairie with B Set or a 57xx pannier with a small 5-wagon goods train,as well as either the auto-train or the railcar. On the On30 side, it can accomodate my Forney with two coaches or a short log or mineral train.

The 1:33 gradient gives me enough clearance to get an On30 train under the terminus, about 6 inches. With the supposition that the GWR line was originally built to the broad gauge, I will have enough space between the tracks in the station area to allow the On30 trains to pass. In addition, broad gauge track allowances on the bridges over the river will accomodate the larger O scale trains

I am aware of the discrepancy in sleeper (tie) spacing between the scales, and I'm prepared to live with that, especially with careful and copious application of ballast. I think that one of my biggest obstacles will be in configuring interchangeable tunnel portals so that my OO trains won't look like they are heading into some gaping hole in the side of a hill.

Any thought on this insane plan? Has anyone else successfully mixed scales like this? Thanks for your input