Freely substitutable; that may be swapped at will.
Eli Whitney's development of interchangeable parts was a breakthrough for modern manufacturing. Prior to that, each part had to be made custom.
Constructional features of the railbus include the use of rubber suspension for axlebox/underframe and underframe/body mountings, the use of B.U.T. power transmission equipment, many components of which are interchangeable with those on British Railways standard railcars, power-operated sliding doors, and an absence of normal sidebuffers or drawgear.1958 April 23, “Diesel Railbus for British Railways”, in Railway Magazine, page 275:
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Sometimes the same words mean opposite things. Sometimes the opposite is true. Shock absorbers are called shocks. Slow down and slow up are interchangeable.1997, George Carlin, Brain Droppings, New York: Hyperion Books, →ISBN, →LCCN, →OCLC, →OL, page 104:
The English, until relatively recently, seem to have imagined “English” and “British” to be interchangeable, as if Britain was just a bigger England.2014 September 16, Ian Jack, “Is this the end of Britishness”, in The Guardian:
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Following each other in alternate succession; alternating.