In response to the original qs:
1. I think the main official rule is that flash photography is banned in tube stations, for the obvious reason that it could surprise a driver coming into the station. If you intend to spend any time in a tube station not travelling, however, it is extremely wise to get permission to take pics. Also, for the sake of those that have to use the network, please be considerate and don't go spotting in peak times or at a major station, as you'll just be an obstacle to them. You can make the odd good spot on the tube, I have seen the LU measurement train pass through my own station at Tooting Bec before, and LU battery locos on various occasions, but they are rare and you have to be vigilant. If the District line is closed, its worth taking the c2c train from Fenchurch Street, which runs alongside, because I've seen the First GBRF/Metronet 66s and LU battery locos out there doing maintenance/engineering. Still never caught anything like this though
http://en.wikipedia.org/wiki/File:Londo ... _Train.jpg.
2. Good stations: All the ones people have mentioned, but perhaps try Willesden Junction, and the small stations up the WCML towards Watford - Pendelinos, LM desiros and others, LO and Bakerloo line trains are visible, as well as frequent freight on from the container depots near Willesden Junct, especially on weekday mornings.
Gospel Oak on LO is also probably worthwhile, and you can use your Oyster to get there - plenty of freight runs over the NLL.
East Croydon is very busy and sees a good variety of units, best to avoid it in peak-time for the same reason as the tube.
Baker Street - the original capital of the Metropolitan Line empire, and the District/Circle platforms are one of the earliest tube stations - also the Met platforms are replete with heritage features, you really could be in an Agatha Christie book there! Also worth taking an A stock train, as they are so old - and go to John Betjamin's 'MetroLand'.
Eurostar are hard to spot, but you could try standing on the Regent's Canal towpath just north of St P's and you will see Eurostars and EMT trains etc. entering and leaving the station. The canal is also worth a walk by the way.
Other good non-station locations - Wandsworth Common, near the station, plenty of ops to see trains passing away from the station.
King Charles Road bridge in Surbiton has only two wire mesh fences as parapets, a spotter's dream (it looks over the SWT mainline!)
Walthamstow Marshes - the junction between the Liverpool Street - Cambridge line and one of Lea Valley branches is here, right in the middle of a public open space - plenty of EMUs and a great view in an attractive location.
Other good London rail routes:
DLR - you have to go on the DLR, sit at the front and pretend you are driving. The Canary Wharf and Airport branches are highlights. Greenwich is nice for the pubs too.
Tramlink - go to Croydon and take some trams, its only £1 with your Oyster card! Again avoid rush hour, and school closing time.
c2c - a great view over the Thames on your way out to Shoeburyness.
East London Line - should be re-opened as LO soon, go through Brunel's original tunnel under the Thames (you can spot the pillars).
Waterloo and City - a two station line, but worth it just for the lulz!
Hope the above helps. There's probably more I've forgotten, but these are good tips. I'll post more here if anything strikes me.
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I would say now, but I'm deadly tired and need sleep for the moment
