A 2-car West Cumberland set 79018/79612 went into departmental use as the Ultrasonic test train, numbered 975007/975008.
One of the single cars 79900 went into departmental use first as a route learner then based at Derby RTC. It became 975010, "Test Car Iris".
975007/8 seen at Reading on 25/7/88.
Eric Stuart recalls one experience about the Ultrasonic Test Train - "I was a driver on the Met at that time and had gone to Moor Park to relieve a driver on an Amersham train. Whilst waiting, the DMU came through at about 15mph, followed shortly by the train I was to pick up. The driver got off and I set off toward Amersham. We stopped at every signal for 5 minutes or so, as the dmu continued its funereal journey. We eventually got to Amersham over half-an-hour late.
Almost immediately, the following 115 to Aylesbury arrived. At that time, the service was hourly and the 'box at Gt Missenden was switched out except in peak-hours. As far as I know, the 115 had to wait over half-an-hour for the test unit to clear the section to Aylesbury or probably to the IB signals somewhere near Wendover.
When I got back to Ricky depot, the SM had been talking to the Controller. It seems the test unit was supposed to have travelled at normal speed to Amersham and then do its test run (15mph or whatever) to Aylesbury. Some wires had been crossed, with lack of amusement all 'round."
M79649 was a DTC that was converted to an inspection saloon for the LMR soon after being built. By June '67 it was at Shields electric traction depot, no doubt for use in conjunction with the South Clyde electrification, and renumbered DB999510.
When the Derby Lightweights were withdrawn from passenger use in the late '60s there was no interest in DMU preservation. Thankfully, the Ultrasonic set and Iris survived long enough in departmental use for this attitude to change so they can be appreciated today.