One thing affecting the fuel consumption on the Challenger, and especially on the Big boy, is their massive weight. I'm convinced this also affects their reliability, since neither last as well in use as their nominal rating would lead me to expect. They're effectively hauling a lot more load than the actual consist, so fuel consumption goes through the roof and they wear out faster. Frankly I never use them unless I have no choice.
Yep, Europe/UK is mainly where they were used, and this one isn't a UK type. There were some tanks that were medium to heavy freight (GWR in particular) but generally they were short haul mixed traffic or express AFAIK. Some were used in Australia for suburban runs where a tender was a nuisance (C30 class being the most obvious example).If it was up to me I would knock the levels of the single Tank down to a little above what it had in 1.06. Tank locos seem a passenger, light freight loco to me. With a double I would see it being a medium duty loco. I think it is meant to be a European design?
The thing is that the niche for which tank engines were used doesn't really exist in RT3. In the game there are no advantages to not having a tender. This means the only reason for having a tank engine of any sort in the game is if somebody just wants to see a tank engine or two running around their tracks, maybe because they're playing a suburban scenario and they think tanks fit that or whatever.