Within hours the graffiti has been updated. ‘I don’t shave for Sherlock,’ is already scrawled across the phone box at West Smithfield by Bart’s Hospital. It’s a line from The Empty Hearse, last night’s episode of the BBC ONE smash hit. The phone box, yards from the scene where Holmes may-or-may-not have fallen to his faked death, is a shrine to Sherlock. Fans from all over the world have paid their respects and made their mark.

Sherlock is a multi-layered drama, and things are not always as they seem. Moffat and Gatiss love playing with the viewer and yesterday’s episode almost took that to new extremes with different theories of The Fall being presented, each of them apparently seriously. There was some anger in our living room (and on Twitter and elsewhere on the web) about the inaccuracy of the presentation of the London Underground.

Sherlock and Moffat’s other show, Doctor Who, have something in common with the Tube: an army of geek fans. Given that Sherlock is itself a kind of fanfic, its portrayal of both its own fans and the Tube’s was both gentle and a little spiteful. Today, though, this Tube nut is not sure what to think.

We won’t go through all the Tube inaccuracies here. First, and to cut a long story short, you need to know that Underground lines fall into two categories: cut-and-cover and deep level. ‘Deep level’ lines such as the Jubilee and Piccadilly use smaller trains. ‘Cut-and-cover’ lines such as the District have full-size trains. The problem is that, for safety reasons, London Underground isn’t too keen to have film makers running amok on its network, and limits filming, more-or-less, to two sections of disused line: the Aldwych branch of the Piccadilly and the Charing Cross branch of the Jubilee – both, as you will recall, deep level.

Our problems start at Westminster. Stinky Mr Shilcott (and why does he have the security tapes at home?) shows the suspiciously-empty ‘last train’. Westminster is on both deep level Jubilee and cut-and-cover District, so Charing Cross should be OK as a set? Well, no. Westminster’s Jubilee platforms have doors on the edge. But in any case we are supposed to be seeing the District line…

Sumatra Road has been dropped rather beautifully onto the ‘old’ maps that whizz through Sherlock’s memory. The real life example of North End (AKA Bull and Bush), built at platform level but never opened, is well known and isn’t remotely in geek territory. If there were an abandoned spur (you’d need more than a station if you were going to store rolling stock) coming off the District line it is inconceivable that Shilcott could have forgotten about it.

I’ll gloss over small issues such as how Westminster to St James’s Park is five minutes on the show but two minutes in real life, and even the small questions of (a) it not being possible to simply decouple a car from a train and (b) how the line controllers wouldn’t notice that a car had gone missing, even if (c) you could engineer a decoupling and a run into a different tunnel.

Does all this really matter? Well, yes and no. For those of us who are looking out for it, any step away from the real world is jarring and ugly. (You could be equally annoyed at the weird presentations of Parliamentary procedure, or the strange route taken to get to Watson’s bonfire moment.) But there is a counter-argument. Sherlock is fictional television. It aims to tell a story. And, frankly, deep level platforms are more recognisably ‘Underground’ than cut-and-cover stations. For people unfamiliar with the London Underground, this presentation makes more sense. And once you start looking at the problem through this lens, even what I felt to be last night’s worst continuity crime starts to look a little different…

Sherlock and John run up towards an apparently abandoned train. In reality, it’s a set of deep level (ie. small) ‘1973’ stock that lives on the Aldwych branch. That’s fine, but when they climb aboard it morphs into a car of cut-and-cover (ie. big) ‘D78’ stock such as you would normally find on the District line. That’s right! It’s a different train! Last night, this was the cause of much harrumphing and tutting and general disquiet.

Today I am more relaxed about it, and not just because – let’s face it – it’s not unusual for exterior and interior sets to be different. I realised that whether they intended it or not, the Sherlock team have given us a shout out that is as beautiful as it is audacious. In the shift from ’73 to D78 stock we see a mode of transport that’s bigger on the inside than on the outside. A drama that’s as multi-layered as this one can get away with referencing its show-runner’s other show. It’s only just New Year but Stephen Moffat has served up your first Easter egg of the season.