In all* the excitement** about the new tube map this week, we forgot to mention something: Transport for London has updated its "all rail services in London honest" map, too.

The "London's Rail & Tube services" map, as it's imaginatively known, is less iconic than the tube map. But it does have the advantage of being basically complete in its depiction of rail-based transport in the London area – even if it insists on colour-coding its main line routes based not on where they go but on which corporate interest currently happens to run them.

In fact, the map is actually more than complete. It's no longer limited by the Greater London boundary, nor even by the M25, nor anything else that rational. Increasingly, it shows those stations in outlying commuter towns from which you can use the Oyster card ticketing system.

So, for a while now, this chunk of Surrey has been included in zone 6:

More recently, this chunk of Essex has appeared, although not all of it gets a numbered zone:

But this latest edition really goes to town. For a start, there's this branch to Hertford:

And this line, which runs all the way to Gatwick Airport, is getting Oyster readers this spring.

For the first time, you'll be able to use your Oyster card on the Gatwick Express (that’s the line shown in brown). It's attained this status before the Heathrow Express, despite the fact the latter airport is actually, y’know, in London, rather than two counties away. (Thanks, BAA.)

We mention this for a couple of reasons. One is that, well, people want to read this stuff***.

Another is that it feeds into one of our favourite recurring debates – that of where cities end. One of the various ways of defining a city is by its "metropolitan area" –the zone from which people will commute to work in it.

The various outer suburban lines being added to this map are all well within London’s metropolitan area. But an extended Oyster zone will arguably extend and consolidate the relationship between the city and its satellite towns, nonetheless. Redhill isn't technically part of London – but armed with frequent trains and Oyster card readers, its residents will have better access to the city than the residents of the rural village of Pratt's Bottom (which, despite being miles from anywhere, is technically inside the city limits).

That said, the piecemeal way in which the Oyster card boundary is being extended beyond the Greater London boundary is making for a pretty odd looking boundary. It excludes various contiguous Surrey suburbs, while including a weird dog leg to Gatwick airport in neighbouring Sussex. It excludes one chunk of Hertfordshire that lies inside the M25 (Radlett, North Watford) while including another (the upper Lea Valley) outside it. Hertford East, on one line, is in; Hertford North, on another, is out.

The result looks like this.

Click to expand.

Which is silly.

But, nonetheless, there we are. That's the new London boundary. Deal with it.

Oh, but only until the next time they update that map, obviously.

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*Our

**Continuing obsessive interest in our never ending search for clicks

***No, seriously, we look at Google Analytics: they really do