There are two ways of looking at a gearbox and it depends primarily on the user. Generally gearboxes are classified by their rations. There are additional characteristics about the various types of gearboxes that are more technical than I wish to get into at this stage.
Suffice it to say we want to narrow down the types of ratios available to the Hobbyist that are direct, simple and easy to follow for the beginner to the expert hobbyist.
We are dealing primarily with Brass Steam Locomotives and the original gearboxes that came with the engine versus replacing those gearboxes with better equipment. Ratios vary from very slow to high speed. Over the last 50+ years the gear boxes have not necessarily been standardized. The general ratios are 27:1 or 28:1 for high speed and 36:1 or 37:1 for low speed. However, some of the locomotives produced in Asia came with odd ball ratios of 41:1 which made the engine crawl at best.
The gearbox alone dose not necessarily make the engine run faster or slower. It depends on the can motors rpm values and torque values. For example, if I wanted a 2-10-2 locomotive to run at scale standard speed with a load I would most likely chose a gear ratio of 28:1 and a can motor of 12.5 rpm with a good torque value. Generally the size of the motor will determine the torque value. Most can motors will fit in a 2-10-2 locomotive.
I am not found of the lower ratio gearboxes but would rather chose a can motor that meets my needs. This is a prejudicial decision and is not necessarily the desire of anyone hobbyist. I prefer the 27:1 or 28:1 ratio gearboxes with the right can motor to control my speeds.
The other ingredient that is necessary for this system to work is the gear located on the main driver of the steam engine. This gear is the same ratio that you would choose for you gearbox. The gearbox actually rides on the main driver gear. Without getting into the particulars of the can motor to the gearbox it should be noted that the motor shaft to the gearbox shaft must be stabilized so the gearbox does not try and spin around the main gear. That is the means of attaching the two together to incorporate the power to gearbox mechanism. In another article I will explain the drive shaft particulars and the types of mechanism that are available to accomplish this task.
It should be noted that there are manufacturers of gearboxes and the like who have catalogs to help you determine the type of gearbox you are looking for. I would like to make it clear that for example, a 28:1 gearbox may have several characteristics about them that make for different uses for different engines. This is where the catalog comes in as very handy reference guide.
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