Real Dictionary

siding
countable noun: A siding is a short railway track beside the main tracks, where engines and carriages are left when they are not being used.
uncountable noun: Siding is a wooden or metal covering on the outside walls of a building.

siding in American English
a covering for an outside wall, as of a frame building, consisting generally of overlapping shingles, boards, aluminum panels, etc.
noun: a short railroad track, opening onto a main track at one or both ends, on which one of two meeting trains is switched until the other has passed

siding in British English
noun: a short stretch of railway track connected to a main line, used for storing rolling stock or to enable trains on the same line to pass