I may have found something suitable for you. There is a small book "Greenley's Model Steam Locomotive Design and Specifications", revised by Ernest A Steel, and published in 1952 By Cassell & Co Ltd. This has a small group of drawings of a Gauge 1 American Pacific Loco with a coal-fired boiler, but possibly dating from around the 1920s or 30s in design, I've just realised!
Gauge 1 is 1.75inch gauge and a scale of 10mm to the foot - about 1/30th full size, so a bit smaller than your 1/12th scale. Can be run indoors but is also popular as a garden railway. There is a very active Gauge 1 Model Railway Association
www.gaugeone.org who celebrated their 60th year in 2007.
The nearest British equivilent to 1/12 scale is the 5inch gauge with a scale of 1 and 1/16th inch to the foot. Going down you have 3.5inch gauge with models built at 3/4 inch to the foot. But at these sizes you are truely into model engineering and passenger hauling capabilities. Best bet is to seek out your local society of model engineers and chat with them about what you want to do. They may be able to give you sources of plans as well. The larger local libraries may also have books on model engineering - look around 625 in the Decimal Classification system at the end of books on full-size railways for the books on models.
There are also several monthly magazines on live steam modelling & engineering that might give you a lead.
Regards,
John Webb