London Blackfriars doesn't have any ticket barriers at the platform entrances/exits. And, unlike say London Waterloo, the exits are rarely staffed. Say on one day in two weeks. There are two exits, a wide one serving platforms 1-4, and a smaller one for platform 5 which is for Thameslink trains going north to Kings Cross/Luton/Bedford. Most of the time, if there is a ticket check, it's only on the wide exit, not the smaller one.

So there's clearly a temptation for passengers wanting to get off there not to buy a ticket.

It's always amusing to see, on days when there is a check, some passengers get off at the front of their train on platform one, head towards the exit, spot that there's a ticket check, then quickly turn 180 degrees and head for the underpassage that links the platforms…

And 99% of the time, they get away with it if they're smart. The London Underground 'carnet' ticket (prepaid single journey in zone one, bought in a pack of ten) is only valid on tube journeys… and Thameslink journeys from Elephant & Castle or London Bridge to Blackfriars including further up to Kings Cross.

So if it turns out that there's a ticket inspection on the smaller exit, and there often isn't, they just wave a carnet at them and claim they've come that way.

(Why can't they do this from platforms 1-4? Carnets aren't valid on South Eastern trains, the other company that uses the line between Elephant & Castle and Blackfriars, which only use those platforms.)

Of course, it would help if there's a Thameslink train on or just leaving from platform five, so it's particularly funny to spot a small huddle of people waiting on the stairs up to the platform, out of sight of the inspectors, so they can later pretend they've just got off a train that hasn't yet arrived.

But today was a 1% day.

"Oh no, a ticket inspection" go the naughty ones and do their 180s to head for the underpass. There was a Thameslink train on platform five too, so no hanging about waiting. Of course, they might not have noticed that this particular train had gone via Elephant & Castle without stopping, and you can bet that the ticket inspectors knew that.

Even better, from the point of view of spectators in the know, there was an inspection in the underpass too, at the base of the stairs to platform five! Unheard of!!

Unsurprisingly, they were having quite a catch.

What the really smart ones would have had to do – and I didn't spot any – was mutter 'oh shit' to themselves, and do a quick 90 degree turn up to platform four. There, they could catch a Thameslink train to London Bridge, so they could then catch a Thameslink train back to platform five. Bingo, the carnet is now valid.

Mind you, they'd have got into work about half an hour later than usual.

I was left thinking that, given that the Thameslink train on platform five hadn't stopped at Elephant & Castle, it'd have been even better to have let the naughty ones try to present their carnets at the platform exit, then arrest them for ticket fraud, rather than just charge them £10 for a penalty ticket before they had had the chance to get deeper into the shit.