You might have seen some new signs popping up around the tube stations recently showing off London Underground’s heritage.

You might have thought they are a nice touch and enjoyed reading them. Some of you might have grumbled that TfL should spend the money on improving the service and not wasting money on showing off its heritage.

But, there’s a very surprising reason for the new posters.

Look carefully at the posters — they all replaced tube maps. Every single one of them fills a space that was occupied by a tube map. Now look at all the other tube maps on the platform… spot anything?

All the other tube maps are behind glass, but on each platform on the Underground, one special tube map was specially laminated and wasn’t put behind glass. The new heritage signs are also laminated and not behind glass.

So why are some behind glass, and others not, and why would it matter?

The secret is in the poster frame.

On each platform, one particular poster frame hinges outwards, and when stations are closed for some reason during operational hours, they display a warning sign to the train drivers.

Now the issue is that the design of the folding out signs didn’t include the glass protective sheet to keep the maps clean, which you might think of as a nuisance, but that’s all? In fact, it turned out to be a surprisingly expensive problem.

Any printed material places in the tube platforms have to comply with a fire resistance regulation known as S1085, which covers flammability, smoke emission and toxic fume emission in case of a fire.

A poster placed behind a glass sheet which is itself rated as compliant is perfectly fine. But a poster mounted directly onto a sheet of metal, with no protective glass would be a problem. Replacing all the flip-out signs with designs that added a sheet of glass would have been very expensive.

So, every time new tube posters are made — which is roughly twice a year — all the paper posters are swapped over and placed nice and snug behind glass sheets. But, a special batch of just a few hundred posters have to be made for the flip-out signs, and they require a specialist production process involving fire-proofing and lamination.

All that work just for one tube map per platform.

And get ready for this… those special posters cost around £21,000 a year to produce.

A few months ago, at one of their regular maps planning meetings, the map makers inside TfL turned to these annoyingly expensive maps, and someone thought, what if we got rid of them and put something long term on those flip-out panels. Then it was thought it would be nice to put up some heritage signs instead.

London Underground has rightly been showing off its heritage in recent years, and people are showing increased interest in tube history. So, plans were put in place, and a series of heritage signs produced, focusing on six core themes.

Maps, Signalling, Station architecture, Trains, the role of women in the company, and the District line (for it’s 150th anniversary).

Over the past few months, the poster fairies have been out and about visiting stations at night and swapping over the old tube maps with new heritage signs. Most of the District line stations got the District line heritage signs, and the rest scattered around the network.

Not all the stations though.

Although 216 stations had enough “maps behind glass” to have the laminated map swapped out for the heritage poster without the public being left bereft of maps, the remaining 54 stations are lacking in existing maps, and removing the laminated map could leave some platforms with insufficient maps to meet standards.

So they are putting up new frames on those remaining stations over the next few months to house tube maps, and they will also then get a heritage poster put on the flip-out panel, just like all the other stations.

So, the rather unexpected reason for all those nice new heritage signs that are springing up like mushrooms across the network — is to save money.

The stations are now adorned with pleasing information, and TfL saves around £21,000 a year. Money that can be pumped back into the network.

And I bet you never expected cost cutting to be the real reason for the range of new posters.