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.

[...] 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.