Siding is material, such as wood, vinyl, or metal, that covers the outside walls of a building to protect and decorate it. In railways, a siding is a short track connected to the main track where trains can park or pass.
“Siding” is formed from “side” plus the noun-forming suffix “-ing,” originally meaning something that goes along the side. The building sense developed as exterior cladding that runs along the sides of structures. The railway sense also reflects a track running off to the side of the main line.
Both house siding and train sidings live at the edges of more important structures—one at the side of a house, the other at the side of a main track. English quietly reuses ‘side’ to mark things that branch off or cover the margins. Once you notice it, you’ll see ‘side-’ compounds everywhere.
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