Ok, I made this one up, I admit it:

A Putney sardine is someone who absolutely must get on the train and squeezes themselves in rudely, even when the train is obviously already full and everyone else is hoping that they don’t have to breathe for the next five minutes. It’s at this point that the Putney sardine will bundle themselves forcefully into the carriage.

Those of you who’ve had the misfortunte to take a peak-hour train through Putney will know what I’m talking about here.