I once had a seat booked going from Runcorn to London, going back to uni in Brighton. Don't know where the train originated from but when I got on it was PACKED.
Got on at the correct end of the correct carriage for my booking, staggered to my seat to find ... a breastfeeding mother, with a VERY young baby. Took one look at her and turned back to sit on my bag near the rancid toilet. 
5 minutes later, guard comes along. "You can't sit there, love". Told him I had a res, but someone was sat there. The guard did what I can only describe as a railway employee's version of a boob hoike and marched down the carriage with my res ticket in hand, a look of steely determination on his face ... 60 seconds later he comes back.
"Follow me!"
I thought I was about to be chucked off the train or something. It was only my second or third foray to London from the NW, and I was still getting used to it all.
We marched our way through 3 carriages.
He stopped. "Sit there - do you want tea or coffee?"
FIRST CLASS?!?! I never wanted that journey to end!! And now, 19 years later, remember it fondly!!