Except one night a year, when anarchy reigns in the buffet car and the space into which about 30 people are crammed would contravene all rules on animal transportation.
Three commuters are organising the annual "Wot a Party" for the Bognor regulars. For £2 you get wine, unlimited Twiglets, and Slade pumping out "Merry Christmas Everyone" through the ghetto blaster provided by DJ Paul (alter ego Paul Heslop).
By day, Gavin Leach works for "a major financial institution". But once a year he transforms himself in a dress-shirt and bow tie. Along with DJ Paul and Vicky Mason, a secretary from Horsham, they leave work early to decorate the train with tinsel, bells and streamers for the commuters' party.
Once a year these people squash themselves into one carriage, wearing party hats, pulling party poppers, trading greetings, insults and drunken confessions. "It's male bonding," said Jim Carruthers. "We like being squeezed in corridors," he tells David Matthews, stage manager for Blind Date.
"That man over there has seen Prince Charles with no clothes on. He was at Gordounstoun with him," he adds. The poor man denies it so vehemently it can only be true, as he spends most of the rest of the evening hissing: "Don't print my name."
Pat Sangster, a businessman who uses the 6.20pm every day, is not shamed that he usually never talks: "I prefer to do the crossword in the mornings. I don't want to chat. Yes, I'll speak to people at Christmas and then never again till next year." So howdoes word about the party get round? "People just know," he says mysteriously.
Strict demarcation lines are laid down for partygoers. Those who sit are "boring bastards"; those who sit playing bridge are "boring bastards who should know better".
Your true commuter stands in the buffet car, sloshing wine over everyone's Burton suit and falling into bowls of peanuts until he falls out at Horsham.
Women tend to keep away. Political correctness hasn't arrived on the 6.20 yet and, after Crawley, hands start wandering.
Impostors are shunned. "I don't know you. I've never seen this man before," yelled a regular trying to manhandle a hapless intruder off at Clapham Junction. "No, I never get this train," admitted Peter Statham. "I just heard there was a party to Bognor on this train and thought I'd gatecrash." As the train approaches Billingshurst, DJ Paul lurches round, trying to collect enough money to pull the communication cord. He only makes £12.
By Pulborough, most stumble off the train with shouts of "Are you driving?" "No I'm a lawyer." The same joke is doing the rounds. "Are you going all the way?" "No, only to Barnham [the penultimate stop]."
By the end of the line there are only the steward and a couple of diehards left to take down the decorations.
They tumble off the train with the faint sound of "He's seen Prince Charles with no clothes on . . . " floating on the air. Tomorrow morning the Bognor line will be silent. Pat Sangster will do the crossword; Gavin Leach will read the FT. And you're leftthinking: "Did this really happen?"