transmission was frequently attributed as being the largely single cause of these issues, while undesirable instances of wheel slippage was another common occurrence. As a consequence, the propulsion systems of not only the Class 144 but its Class 143 and Class 142 siblings were replaced after only a few years of service. This refit not only resolved much of the unreliability issues but also gave the Pacer greater acceleration through the installation of a more powerful engine. Privatisation era Since the privatisation of British Rail, the Class 144 has been operated by several franchises. The first private operator was Northern Spirit, which was later taken over by Arriva Trains Northern; the latter opted to refurbish its fleet between 2002 and 2004 to enable its continued service. Post-refurbishment, the units emerging in a new silver and red Metro livery complete with refurbished interior. In December 2004, the fleet was transferred to the then-new Northern rail franchise. Northern Rail replaced the silver and red Metro livery with Northern Purple and Blue. By April 2010, all Class 144s have been repainted into this new livery. Around 2010, Northern Rail subjected their fleet of Class 144s to another refurbishment programme. 144006 was the first unit to be refurbished, it had also the first one to be refurbished in the joint Arriva Trains Northern/WYPTE Metro programme in 2002. The refurbishment enhancements involved numerous interior changes, including the installation of 'easy to mop' flooring, an extended area for bike storage at one end of the car by removing a bulkhead wall and extending the perch seats from three to four, repainted hand grips and stanchions, new dado side panels and repainted wall ends, retrimmed seats in the purple Northern Rail moquette, a repainted ceiling, and repainted driving cab. During April 2016, Norther Rail's Class 144s were all transferred to the new franchisee Arriva Rail North; in turn, Northern Trains took over operations of the fleet on 1 March 2020. Rail Vehicle Accessibility (Interoperable Rail System) Regulations 2008 and the subsequent Persons of Reduced Mobility - Technical Specification for Interoperability (PRM-TSI) require that all public passenger trains must be accessible by 1 January 2020. As originally delivered, the Class 144 does not meets this requirement and has to be withdrawn without modifications to become compliant. During the 2010s, the rolling stock leasing company Porterbrook proposed an extensive refurbishment of the Class 143 and Class 144 fleets with