wave out of the window at a stranger on the platform.

Do this with a really genuine friendly smile until they're convinced they know you but have forgotten where from...I mean you're travelling by yourself it's not like you're showing off or taking the piss out of them.

Do it until they wave back.

Then as the train pulls away drop your smile, stare them right in the face and do the 'Finger-Slicing-Throat' gesture

Dare you.