Track is sold in sizes called “codes,” which indicate the rail height in thousandths of an inch. Code 100 (.100″), code 83 (.083″), and code 70 (.070″) are common sizes in HO scale while code 80, code 70, and code 55 are used for N scale. These sizes represent the various sizes real railroads use.
You’ll also be faced with choosing brass, nickel-silver, or steel rail. At one time nearly all HO track was made with brass rail. However, the yellow color isn’t realistic and brass must be kept quite clean for efficient electrical contact. Nickel silver is a similar soft-metal rail with a more realistic silver-gray color and better electrical conductivity. Steel rail is used in Bachmann’s flextrack and its color is perfect, but the rails are difficult to cut and file.
Small flat metal sleeves called rail joiners are used to connect sections of scale track. Molded plastic rail joiners are used for electrically insulated joints.
From the book Trackwork and Lineside Detail for Your Model Railroad from Model Railroader Books.