specially equipped Mark 3 coaches. Mark 3s remain in service as part of HSTs with CrossCountry, Great Western Railway and ScotRail much as they have since introduced. These were refurbished when their operators were privatised, with all except East Midlands Trains' receiving new seats between 2006 and 2009. The introduction by Virgin CrossCountry of 220/221 Voyagers in the early 2000s rendered many Mark 3s surplus. After periods of storage, all have now returned to service with other operators. The introduction by Virgin Trains of Class 390 Pendolinos in the early 2000s resulted in the withdrawal of the locomotive hauled Mark 3s. Some were cascaded to One to replace Mark 2s on the Great Eastern Main Line, while a few replaced Mark 2s on the Night Riviera. Most were placed in store at Long Marston. Many have since been returned to service, both as locomotive hauled coaches with Arriva Trains Wales, Chiltern Railways as well as being converted for use with HSTs by Grand Central and CrossCountry. This required modifications to the coupling mechanisms and electrical systems to make them compatible with Class 43 power cars. Mark 3 sleeping cars are used on the Night Riviera services. Virgin Trains retained one complete Mark 3 set. It was initially used to cover peak-time London Euston to Birmingham services while the Pendolinos underwent modifications, but later covering the loss of 390033, written off after the Grayrigg derailment in 2007. In July 2009, it was refurbished and repainted at Doncaster Works in the same style as the Pendolino and Voyager fleets, but with the British Rail seats and interior fittings retained. This set was nicknamed the "Pretendolino" by enthusiasts. It was transferred to Abellio Greater Anglia in November 2014. In 2016 it moved to TransPennine Express to be used as a crew trainer in the lead-up to the introduction of Mark 5s. Sewage discharge Legally in the UK, train operators are allowed to discharge up to 25 litres of untreated waste at a time on to the track and the discharge from each toilet flush is considerably less than this. Most Mark 3 carriages have no retention tanks, discharging onto the track via a U-bend/pipe near the bogies, and in the 2000s both the RMT trade union and politicians were concerned at the environmental impact of this legacy issue. The problem was first raised in 2003 after Railtrack staff at abandoned local clean-up and