(28 Feb 2015, 12:33 pm)DaveyBowyer wrote But a heavyweight vehicle bar a few lecky and build issues, would have no major trouble. Several of the 61 reg DB300 deckers at Blyth and Belmont have had engine trouble and the haven't been always used on express work but the 7x 11 reg B9s at Washington have dealt with a more demanding route (about the same length as the X21 / X22 between Newcastle and Ashington) and have suffered from no issies often with full loads between Newcastle and Galleries combined with rough driving to keep the service running as close to time as possible.
Heavyweight buses are the past, Euro emission levels are best met with a smaller heavier engine hence the lightweight chassis - so they still meet TfL regulations.