Abstract: A rail system for a modular reptile and small animal cage comprises a plurality of squared elongate channels, each channel having an X-shaped cross-section that includes: four arrowhead tips pointing to the corners of a square with a gap between adjacent arrowhead tips defining a slot for holding a cage side panel or a cage accessory. When combined with a plurality of side panels, bottom and top panel, these slotted elongate channels can be used to make an expandable, modular reptile/small animal cage. A method for making such cages is also disclosed.