Buying the models can be quite expensive. There are several basic sizes of model railroads. The size depends upon the scale and gauge. Scale is the proportional reduction from full size to model. Gauge is the distance between the inner surfaces of the rails.
Z scale is the smallest size.
N scale is popular among modelers who do not have much space. The TT-scale is no longer commonly used. The greatest variety of engines, cars, and other supplies is available in this size, often referred to as HO gauge. This scale offers some of the advantages of the larger size with the maneuverability of the smaller. The larger size of O scale allows the builder to add intricate detail to the engines and cars.