Lox mentioned in one of his responses the thorny issue of how to keep the seat next to you empty.
Many of us that travel the railways will at one point or another just want a bit of space rather than sitting next to someone you don't know and be forced to actually touch another member of the human race... Imagine the horror as someone sits down next to you and either:
a) sits on the bit of your coat that has accidentally on purpose been left on the other seat
b) actually is large enough (as I am) to wedge themselves into the seat forcing your thighs to be in such close proximity that they are touching...
There are then a variety of methods for keeping the seat next to you free...
1) Place an additional bag on the seat and tell anyone that asks that the person has just popped to the loo in a bit of a rush.
2) Get tattoos of things like daggers and pagan sacrifices on your forearms and up your neck. Add to this a menacing look that tells everyone you eat human flesh for breakfast lunch and tea.
3) Adopt the "I'm too posh to sit next to anyone" look which is instantly unnerving but not always guaranteed to succeed.
4) Put a copy of the sexual offenders register on the seat next to you with a pen on top of it as if ready to sign...
5) Start talking about Jesus in a loud voice.
There is also the other side of this where we all know we pack ourselves into sardine can like trains and every bit of space is precious.
It is therefore unacceptable for anyone to grunt at the polite enquiry as to the status of the adjacent seat "Yeah, I s'pose so". Have some manners.. Unless you are using any of the 5 methods described earlier..