I'm a bit late to the party but, often I don't sign in unless asked as they can be very, very awkward.
At Liverpool St I was asked for ID, I'm not old enough to drink or drive, why the heck would I take my passport 90 odd miles with me, just to show some bloke? They're not cheap to replace! When I explained and I said "It is for my photography project, i'm going to be there literally 5 minutes and i'm not suing a tripod." He refused due to my lack of ID.
The last time I was there I put my travelcard in the machine, and went and photogaphed some DVTs

and spoke to the driver, he didn't seem to mind how I got in.
Signed in a Paddington, specifically requesting to go behind the barrier line, made me hand over my address, phone number then said I need a valid ticket t pass, so I asked the bloke at the gateline and he seemed happy to let me pass! There was a a manager walking up and down the Platforms, he didn't stop me once.
Had a fright at Euston, was photographing 390s, when a bloke in a big VT coat came running towards me said "Are you getting on this train?"
I said no and he said I am ok, he was just checking I wasn't trying to get passed the manned gateline that would shortly commence.
The HS1 domestic people are always happy to help, "No flash mate" is all they have to say.
I used my Travelcard again at Waterloo, was photographing a 444 when I got quizzed, and told not to step foot outside the canopy? Went onboard a 444. as I wanted to photograph the buffet, and a cleaner apologized for 'being in my way' which was nice of him
Kings cross is an odd place, I've been up and down it all day freely, had a gateline assistant with poor english, when asked could I cross the barriers he just kept replying "Harry Potter not here"! But eventually let me through. Been in a 91s cab, that was fun, and had a guard save me 30p by saying the use of the MKIVs toilet was fine by him, as he had 10minutes til departure. A GC driver honk his horn, scaring my friend to the point where she jumped out of fright, I have a picture of him giggling in his cab somewhere.
Then last time, I got shouted at by a cleaner! Just for being in a vestibule.
Has anyone noticed the gateline staff at Marylebone have been told to shape up? I keep being asked to sign in. I went to the help kiosk where the station manager (who is very nice) said how can i help, to which I explained and he said "tell them I said you can go through" so no problem, except the Banbury 67 had already left!