According to the National Model Rail Association, typical model trains range in scale from O being 1:45.2 to Z being 1:220. For comparison, O scale was formerly known as 1/4-inch scale before scale ratings were converted to letters.
Other typical model train scales include: O, On3 or On2 which equals 1:48 scale; S or Sn3 which equals 1:64 scale; OO which equals 1:76.2 scale; HO, HOn3 or HOn2 which equals 1:87.1 scale; TT or TTn3 which equals 1:120 scale; and N or N3, which equals 1:160 scale. The most common types of train scales are the HO and N scales. According to the National Model Railroad Association, the S scale of trains is also growing rapidly.