When it comes to 'grim railways', as long as the train is on time, seats reasonably comfortable, and accessibility is good (i.e. not having to take 10 minutes to get out of the station), it doesn't really bother me. In fact, I think it gives it a bit more character. While many station rebuilds/refurbs are really impressive, a lot are just clinical and boring. Although I'd definitely like to see less graffiti/litter on some lines.
I've been around a lot of West London today, and was surprised how many lines felt like going back to the 1990s. On the Chiltern line between Marylebone and West Ruislip, the stations looked very uninviting and bland, and didn't seem to have any passengers waiting! West Ruislip was particularly deserted, and the size of the platforms was definitely overcompensating. The Central Line to Greenford was very bland too (although some art-deco stations which I enjoyed). The Greenford Branch was another throwback, feeling like the 1990s. The stations on the mainline into Paddington are not much better.
I agree with Tottenham Hale to Waltham Cross - incredibly bland. The lightly used Angel Road station looks horrible. I sually find "grim" areas really interesting with their own character, but the Lea Valley is quite boring too. Although many of the stations look a lot better since some were taken over by London Overground.
Bethnal Green (National Rail) Station also springs to mind. I grew up on the Chingford-Liv. St line, and remember as a child finding Bethnal Green 'scary', just from looking out the window. I think its the 'shipping container' style waiting room / platform access that give me that feeling!
I do remember the North London Line and GOBLIN line in Silverlink days - grim, particularly on the N. Woolwich branch (but still interesting!).
I've seen the Finsbury Park - Moorgate line (Northern City) mentioned - i'm surprised such a time-warp still exists in central London. While there are a few time-warp areas in suburban London, this seems to be the only grim/creepy/time-warp line left in central London. It contrasts well with the many shiny clinical stations in central London. The actual platforms seem to be clean, but the staircase at Moorgate desperately needs re-tiling, the damp smell needs to be sorted (I strangely quite like it, but it can be very off putting for many), and the walls opposite the platforms on all the stations need a clean - most of the adverts can't be seen properly its so grimy! I like seeing all the NSE signage - its almost like a museum. But, it still looks clean and can be read clearly - I usually have a 'if it ain't broke, don't fix it' mentality when it comes to signs. It annoys me when stations are repainted / signs replaced every time a franchise changes hands - its a waste of money. Not sure how the Northern City has escaped having it all replaced. I'm told the atmosphere of the Northern City is like how most Underground lines felt in the 70s/80s. I find it interesting to see this throwback still existing. I wonder how much longer it will last?