Trains ceased running when Vivarail, the company that built and maintained the Class 230's, went into administration.
Passengers have since complained that the replacement bus services were "too slow" and journeys were now a lot harder for children and commuters.
London Northwestern Railway (LNR), who operate the services are replacing the Class 230 trains with Class 150's, transferred from Northern Trains, say that training on the Class 150's still has to be completed.
Three Class 150 trains had arrived at its Bletchley depot in July and had the LNW livery applied to them, replacing the existing Northern branding.
Since then its technicians, who would maintain them, "have been busy familiarising themselves with the trains", it said.
The operator has taken over from Vivarail, which employed the maintenance staff who fuelled, checked, serviced and repaired the Class 230 trains.
|Above & below vivarail Class 230 previously operated the service|
As a result of administrators being called in, LNR previously said "it was no longer safe to operate our Class 230s... and we were left with no option but to suspend the service".
|The Marston Vale Line runs between Bedford & Bletchley|
Grant Thornton was appointed administrators of Vivarail on 1 December.
Jon Roden, joint administrator, said: "It has not been appropriate to continue to trade the Vivarail business in administration, and we have retained a small team of employees to assist us with realising best value for the company's assets and winding down the company's operations."
Vivarail, was founded in 2012 by UK entrepreneur Adrian Shooter. The trains utilised the bodyshell and bogies of the London Transport D78 Tube stock as part of Vivarail’s commitment to useful life extension and saving waste, energy and materials.
The trains were essentially brand new with everything replaced or upgraded, depending on customer requirements.
In 2021, the company provided a demonstration train at COP26 and its train became the first emission-free train to go over the Forth Bridge.