Definition of "running in"

For years past Pacific locomotives of all classes have worked between Kings Cross and Cambridge, […] both in order usefully to fill in gaps in their long-distance rosters, and also for running-in purposes after overhaul. Before the war the 7.18 a.m. slow from Kings Cross to Cambridge was a regular running-in turn for express locomotives stationed at Kings Cross shed.

1945 July and August, “The Why and The Wherefore: L.N.E.R. Pacifics at Cambridge”, in Railway Magazine, page 242

[...] moreover, there are times of pressure when, to expedite deliveries, cars may be driven in what should otherwise be the running-in period at speeds that do them no good - and over long distances too.

1960 June, “British cars go by rail: I-The L.M.R. wins new Anglo-Scottish traffic”, in Trains Illustrated, page 335