Hornby produce 2 different ranges of set piece (fixed radius curves and straight length) track systems. Both are sold as single pieces, or as track packs that easily help you build up a preplanned layout that combines with the tracks you get in the Hornby train sets.
OO Gauge is Hornby's core range or both trains and track, and has had the longest support and largest back-catalogue. It features 1:76 trains, running on 16.5mm gauged rails. The rail is profiled at Code 100, and is fully compatbile with Peco's Code 100 OO/HO track range.
TT:120 Gauge is the newer of Hornby's track/ train ranges, featuring 1:120 scale trains running on 12mm gauged rail with a Code 55 profile. It is fully compatible with Peco's range of TT:120 track.