The writer is the FT’s architecture and design critic

The London Underground is one of the great exemplars of urban branding. Along with the New York skyline and the Eiffel Tower, it exemplifies the city through a language of familiar signs and symbols: the roundel, the early modernist lettering, the designs of the stations from the oxblood red tiles of Leslie Green’s Edwardian street frontages to the Art Deco suburban wonders of the Piccadilly Line and, of course, the Tube map itself which remains the tool through which many Londoners understand the city.

That map has just a little more colour now, with a dash of purple for the new Elizabeth Line running through its centre and forking out at the periphery. The simplicity of that graphic line belies a £19bn construction project. It has churned up the city across three decades, seeing the destruction of big chunks of London above ground and the creation of cavernous volumes and snaking tunnels below.

Trains have been regularly running through those tunnels since last year. From May 24 they will be populated by passengers. It is an incredible feat of engineering. Winding between the foundations of skyscrapers, under the Thames (and through layers of history which made this project the nation’s biggest archaeological dig), 73 miles of track and 26 miles of newly-built tunnels stretch from Berkshire to Essex.

During my preview, it was almost eerie to glide down the empty escalators at the vast new station at Paddington — the trains move but there is no familiar rush of air as they slip in behind full-height glass screens. They open silently. Everything is clean, everything works. The signage is neat and clear. The station itself, a huge concrete box, is grand but austere.

The last big Tube project, the Jubilee Line’s eastern extension (opened in 1999) was characterised by powerful, distinctive stations, each designed by an individual architect in flamboyant style. But the days of the iconic building are over. The Elizabeth Line’s stations are functional, raw and unfussy but no worse for it. Below ground, the stations are expressed as smooth tubes swerving around corners in dynamic, complex curves. This smooth form is dictated by the sprayed concrete linings of a tunnelling method which looks very different to the old cast iron tubes. The effect is calm but a bit beige — ads are confined to screens.

There are echoes here of older glories. At Paddington, the domed concrete ceiling slabs with inset bronze lights evoke the designs of Charles Holden’s 1930s Piccadilly Line works, the diamond grid ceiling structure at Farringdon has hints of Westminster and the jewels of nearby Hatton Garden. But overall the architecture remains restrained.

The quality, however, drops off the further out it goes. Ilford and Romford, in suburban Essex, look grimly utilitarian and it doesn’t get any better to the west. This is a lost opportunity; central London already has its identity but the rapidly-changing outskirts could have used elegant stations to set the standard for developers.

The opening will trigger a fresh wave of resentment from the rest of the country where, you might suggest, £19bn could have made a real difference to everything from bus routes to cycle lanes. There will be grumbles about an expensive route for the bankers in Canary Wharf to get to Heathrow. The system’s designers point out, though, that the line’s construction has been a boon to manufacturers from enamelled signs made in the Isle of Wight to steel from Cardiff.

Some subterranean projects can alter the city above ground very little; this was not one of them. The West End from Tottenham Court Road to Bond Street has been under construction for years, the wonderful Astoria venue was lost and the replacements are forgettable at best, hideous at worst, garish commercial monsters in gold, pink and too much glass. The city has densified over the new stations, big new buildings from Bond Street to Woolwich have appeared — all this is changing the skyline.

Only weeks ago, images of tens of thousands of people sheltering in the Kyiv Metro were ubiquitous. People similarly huddled together for shelter in the London Underground against the Blitz. But Kyiv’s Metro was designed self-consciously as a palace for the people, lavish, rococo, piled high with art and crowned with chandeliers. London’s Underground was always more practical, a sensible although groundbreaking cumulative work — the first underground railway, recently catching up with newer, shinier, more high tech versions.

The Tube is a functional thing, transporting people, but it also defines how we work or play. It can alter how we use, see and understand the city. Such a project is not just a feat of engineering. As the way its symbols have ingrained themselves into our reading of the city illustrates, the Tube is culture too.