Arguably (AFAIK) the most popular scale, or gauge, of track in model railroads. Any model train you buy in KB Toys is probably HO scale. I don't know exactly, but the HO track is about 1.5 inches wide and each car is from 6-10 inches long.

Addendum/Correction: HO gauge, according to the Lionel web site, is 5/8" wide, making the trains 1:87 scale. I guess my memory is from holding them with smaller hands. Damned perception!