General Considerations

When one starts searching the internet, it becomes obvious there are many designs of miniature locomotives to choose from, especially if you are willing to consider British style locomotives. Before picking a specific design there are other "general" things to consider.
- Scale/Size/Gauge - These three things are interconnected, each effecting the other.  I am most interested in those sized locomotives that are capable of hauling people. The main sizes that fit into this category, are 3/4", 1" and 1.5" scale. The corresponding gauges for these three are 3.5", 4-3/4" (5" in the U.K.) and 7-1/4" or 7-1/2". The smallest size, 3/4" scale, usually is only capable of hauling the engineer and one or two other people at most. The largest scale, 1.5", can haul many people, but, as one can imagine, is the heaviest.  The weight difference between 1" scale and 1.5" scale is not 1.5 times. It is more like a cube function. So, for a 1" scale locomotive that weighs in at 120 pounds, becomes approximately 400 pounds in 1.5" scale.
-A simple project for a first time locomotive should be considered. Large or small, will take about the same time, but, a complicated and detailed locomotive will always take longer than the simple one. A project that takes too long to complete, may never be completed. A long term project is better for a second time build. You will at least have something to run in the meanwhile.
-Machine tools - Do you already own a few machine tools? Good machine tools are not cheap and tooling up for such a project will be expensive. Using tools in a shop other than yours will be a real handicap and will make the project considerably longer time wise.
-Shop Capacity is something folks don't always consider. Just because a finished locomotive will fit in your shop, the manufacture of the parts are sometime beyond the capacity of your machines. Large locomotives require serious machine tools and often overhead lifting facility.
- Cost - Castings and metal stock is not cheap. The larger the locomotive, of course, the more the raw materials will cost. 
-Transport - How will you transport your locomotive to a track? An average car will not allow for a 1.5" scale locomotive, perhaps a large SUV. You will probably need a pick up truck, and/or a trailer to move a locomotive and rolling stock. For 1" scale and smaller, these  will easily fit in the back of a small SUV, or, in the trunk of car. If you want to bring a locomotive, a riding car and a little rolling stock, a small SUV, van or pickup truck is the way to go.
-Proximity to a club with your chosen gauge is important. Also, is the track of your gauge a simple circle, or, is it more complex? The club I chose is actually farther than the closest 4-3/4" track because it is the more interesting than the closer track. Club tracks should offer you something that you could never have in your backyard.
-Experience - What is your machine shop experience? If you have never made anything with machine tools before, it might be a good idea to build a simple stationary steam engine from castings or bar stock before committing to expensive locomotive materials.