A rabbit warren layout is a model railway layout. A group of designs, more than a single constructed layout, rabbit warrens provide a humorous display of continuously moving trains that appear to pop in and out of tunnels, seemingly randomly.

OO9, often also denoted as 009 or 00-9 and commonly pronounced as Double-Oh Nine, is a model railway scale and gauge combination of 4 mm scale and 9 mm gauge tracks, which models a prototype track gauge of 2 ft 3 in. It is a common choice in the United Kingdom for the modelling of narrow-gauge railways whose prototype gauges lie approximately between 2 ft and 2 ft 6 in. The 9 mm track gauge is used by N gauge model railways, a common commercial scale, which means that a selection of wheels, track, and mechanisms is readily available.