Train Sets Only and the National Model Railroad Association both offer a model train gauge guide. Both gauge guides list all common track sizes and include model-to-prototype size dimensions for each.
Though frequently interchanged, the terms "gauge" and "scale" aren't exactly the same. Scale is the ratio of the model to the real object or prototype, while gauge is the distance between the rails. The larger the scale, the more room required for an extensive layout. HO and N scales are very popular, and they have a large selection of products available. The N scale offers the opportunity for long trains, while HO is the better operating scale for way freights, local runs and yard-switching operations.