The first eight sets, the hydraulic transmission red triangle ones, entered service in the West Riding, mainly on Leeds to Bradford services. The British Pathe website features a period newsclip of one of these sets.
The next batch were for West Cumberland, operating on three lines: Carlisle to Siloth; Workington, Cockermouth, Keswick & Penrith line; and the Carlisle - Maryport - Workington - Whitehavem line.
They then started appearing around the country: in Lincolnshire working to Lincoln, Cleethorpes, New Holland, Skegness & Boston; in East Anglia working from Norwich to Dereham & Wells, Ipswich, Yarmouth, Cromer, Sheringham & Holt, Lowestoft, and Dereham to Kings Lynn. From Newcastle they worked to Middlesborough (the 4-car sets) and Carlisle, from Birmingham to Sutton Coldfield and Lichfield, In North Wales on between Blaenau Ffestiniog and Llandudno and Bangor and Amlwch. The single cars worked the Buckingham to Banbury line, and into Bletchley. Stratford sets worked the Romford to Upminster and Wickford to Southminster services.
Eric Stuart adds: " Trains Illustrated repoted that some sets - presumably the Red triangle units - were tested on the routes radiating from Queensbury, near Halifax. These lines had recently been closed, giving rise to rumours of re-opening.
The Yellow D units were used for the John Peel Land Cruise, which I think had various starting points and did a circular tour around the Lake District, including the CK&P line.
A set (3x2?), presumably ones based in Brum, did an excursion (to Bournmouth, I think) over the S&D, probably one of the few dmu runs over that line (for which, in passing, I think they would have been very suitable). Allegedly, an ex-railwayman reported the train as being stock that had "run away," as he saw it moving downhill without a loco at the front!!!"
Eric found an interesting reference in the July 1963 Railway Observer: "BLETCHLEY - On 18th May... M77900, the usual single-unit railcar on the Buckingham branch came in on the 3.11pm arrival towing a delapidated cafeteria car M261M."
A down train arrives at Subdury on the 1st June 1968. David Underwood.
Not all branch lines gained enough traffic to escape closure, and with such a large DMU building programme it was realised that many were surplus. As the Lightweights were non-standard they were destined for early retiral, with the last withdrawn from passenger use in 1969.