Very occasionally, a loud and bright train crawls past my window in the middle of the night (I look out over the line out of London Bridge, just before it crosses Southwark Park Road).
I don't mind it waking me up at 4am, it's very rare and the railway line was there when I moved in (unlike the 24 hour shop downstairs and its antisocial staff, but never mind...) but I'm curious as to what it is.
I'd say it took about 20 minute to do the couple of hundred yards from appearing over by St James Street to disappearing off to the bridge. Maybe two or three carriages long, lit, with windows, a little taller than a normal train, and silvery/grey in colour. Very loud, and bleeping a bit.
A colleague suggested hazardous waste, but I don't see why that would be so slow or so noisy - and certainly not why it would have windows. My theory was that it's some sort of "vacuum cleaner train" run down now and again to get debris off the line, but I've never heard of such a thing before.
Or it's just something I dream now and again and I wasn't awake at all...
I know that train! Not quite as nice and shiny as the one in james' link, but defintely yellow-y coloured, crawls along and is very noisy, ususally accompanied by a gang of workmen shouting very loudly to each other - presumably clearing rubish off the tracks. On the plus side it only happens a couple of times each year. I once loked out the window to see what was going on and the train driver smiled and waved at me - weird.