(rail transport,UK) A principle in railway signalling where only one train or locomotive is allowed to run on a section of single-track line, such as a branch line, at any one time.
1941 July, “The Isle of Man Railways”, in Railway Magazine, page 315:
There are eight 4-wheel coaches of the open toastrack type, but provided with roofs, and during the holiday season, usually beginning at Whitsuntide, trains are run as required on the one-engine-in-steam principle.
1960 March, “Talking of Trains: S.R. branch report”, in Trains Illustrated, page 135:
A change seems likely on the Axminster-Lyme Regis branch [...]. We hear also that one-engine-in-steam operation will probably be introduced on this line.
