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For seeing freight trains in London: there is quite a busy freight route used by trains coming from the Channel Tunnel and heading for other parts of Britain. This route runs through Nunhead, Denmark Hill, Clapham High Street, Wandsworth Road, and Kensington Olympia. It's busiest with freight traffic between about 1000-1500. You can also see engineering and maintenance trains. I'd suggest the best location for photos is probably Wandsworth Road station.
For passenger trains you can try Clapham Junction which is the UK's busiest station in terms of number of trains. Hundreds of trains going through there all the time. Most are quite boring unless you like multiple units though....