Euro 6 buses are the ones with the latest clean engines; there's been a move towards converting buses to euro 6 and ensuring that new vehicles conform - see
http://www.dougjack.co.uk/bus-industry-euro-6-emissions-limits.html for further technical stuff.
It makes utter logic with a reduced vehicle fleet needed to run only the recent, cleanest ones - good for the climate, good for the passengers, good for the
PR▸ .
First has just bought a fair number of gas-powered buses, originally for MetroBust, but now for "ordinary" buses (ie, those without a letter "M" before the number). Do you know if these are included in the Euro VI spec - is it generic, or just for diesels?
Reading the part of the report in the link about selective catalytic converters, larger cooling systems, exhaust recirculation etc does rather suggest that electric vehicles, discussed elsewhere, must have an advantage somewhere. I wonder what the "amortisation" figures look like for a battery bus, a trolley bus, and a tram?