No formation details found.
In pre-electrification days, passengers alight from the 16.15 from Bedford to St Pancras at Harpenden Central on 9 June 1971, which is formed of two of the 4-car Derby/Rolls-Royce suburban diesel units characteristic of this line (CD611 of Cricklewood depot leading). Note the "red triangle" coupling code symbols on the front Tom Burnham.
Do you have any images of the set you can add?