This isn't designed to replace every stop on the line, but to allow local services on an end-to-end line. The vast majority of passengers on that train are going from Big City A to Big City B. The little shuttle is for people getting on and off at small towns along the way. People traveling for those local destinations board the big train on the carriages that end up underneath the shuttle car in order to board it.
They're mandatory in any run of cubicles over a certain size, by the building regulations. It's a fairly recent rule, so you don't see them everywhere.
Friend of mine visited Afghanistan and Iran (he had a Afghani/British friend out there) a couple of years ago. Absolutely amazing places - geographically and socially. I edited his camcorder footage, which I'd link to some stills of if I still had a copy >_<
Yeah, we have mechanical street cleaners in the UK too, but they're not really appropriate for tight London streets. You'd soon get it stuck between a lamp post and a shop stall, for example.
If you're worried about the environment, go for charcoal, as it will very likely come from a managed forest and therefore be carbon neutral. Gas is a fossil fuel and therefor contributes to the greenhouse effect. Not that bbq fuel should be anywhere near the top of a list for cutting down on carbon emissions!
http://funini.com/train/finland/imgs/tram.jpg
But this:http://download.introversion.co.uk/mirror/introversion/blog/20081223/test2.avi
Is amazing