the bletchley class 121 flying meccano units were not permitted to run as single units was due to their prone abilty to break down in service, it used to cause no end of problems, and they could be banged together with most of their 1st generation Cousins.
i used to use the line daily for years and the units worked hard, they were supplemented by 117 twin sets i recall
i seem to recall that the 121 trailer cars built for use by them made the bubbles slightly underpowered.(subject to confirmation)
you have to give the boys at BY credit for keeping them running at all at the time....they were tired out and overdue a well deserved retirement, they should have been replaced by class 150 sprinters years before !!
when they left bletchley and the resignalling happened one of the nicest and friendliest branch lines in the UK changed....the branch is sterile now and to me is just another railway line...