The red of the train looks good against the somewhat drab setting, selective colour would probably work well

There must be scores of places where the tube pops up into the central London scene, most of which will be like this - grubby areas behind backs of buildings. Although I am very happy living on the south coast now, I do sort of miss the character of the grittier parts of London, having spent all my working life just south of the Thames.
I am not a fan of colour popping, although turning the background into a high contrast mono image might look good.