Tandem engine \Tandem engine\ A steam engine having two or more steam cylinders in line, with a common piston rod. Tandem \Tan"dem\, n. A team of horses harnessed one before the other. ``He drove tandems.'' --Thackeray. {Tandem engine}, a compound steam engine having two or more steam cylinders in the same axis, close to one another. {Tandem bicycle} or {tricycle}, one for two persons in which one rider sits before the other.