“You say to a brick,” said the great American architect Louis Kahn, “‘What do you want, brick?’ And the brick says to you, ‘I like an arch.’” Well, yes, so it did, but that was in the last century. What a brick now likes, to judge by the southern side of the £1bn makeover of London Bridge station, of which the second of three phases opened last week, is to be a thin, stick-on film, a sort of beige decal, on the flank of a vast concrete-and-steel construction. It likes to be decorated with sub-Victorian mouldings shaped out of doughy gloop, and to be oddly crowned by a hi-tech canopy of laboured jauntiness. Or at least this is what brick is being told to be. The things that like to be arches turn out to be skinny pieces of curved, precast concrete, jointed in such a way that they are nothing like the voussoirs and keystone of a real arch and so give no sense of carrying load.

When different batches of brick come out of a kiln, they sometimes do so in different shades, and it is good practice to mix them before they’re laid in order to even out the differences. This hasn’t happened here, with the result that a wall that is meant to be evenly toned ends up, like an imperfect suntan, with two-tone bands of lighter and darker. It’s also nice, when putting something such as a horizontal sign inside the opening of an arch, to make it align with the points from which the arch starts to curve in from its vertical supports. Here, the target is missed by some distance.

It would take a small cost in thought, possibly zero, to have made the new London Bridge exceptional

The whole is an eighth-hearted tribute to the sturdy Victorian railway vaults – sculpted, hollowed-out, cavernous – that used to stand here, and some of which remain in silent rebuke, off to the right. The inevitable array of anti-terrorist bollards that stands in front looks more architecturally convincing. It is more embarrassing for standing next to the base of the Shard, whose detail is more precise, crisp and purposeful.

Which is a shame. The country’s major railway stations, public places experienced by millions every day, deserve more, and the architectural practice Grimshaw, which is part of the conglomeration of businesses credited with creating the station, can do better. Its Southern Cross station in Melbourne, Australia, has a compelling, curvaceous oomph, as did its international terminal at Waterloo, which served the Eurostar trains until they moved to St Pancras in 2007.

The anti-terrorist bollards that look more convincing than the ‘laboured jauntiness’ of the station entrance at the foot of the Shard, ‘whose detail is more precise, crisp and purposeful’. Photograph: Andy Hall/The Observer

There is also much to like about the new work at London Bridge, which was always one of the crabbier and more awkward stations in the country. A large new concourse is created at street level, which should make it easier – even if early reports are of “utter anarchy” – to get into and around the station and up to the platforms raised on viaducts overhead. It will accommodate the new Thameslink service, which is the occasion for doing the works. The capacity will increase from 56 million to 70 million passengers a year. There are extra platforms. A work of great scale and complexity has been achieved while keeping the country’s fourth busiest station running, which is not easy.

There is some of the spirit of the Melbourne version of Grimshaw. There is splendour in its abundant volume and in a great plunge through the centre between the viaducts, up which rise long escalators and a great structural steel V. There are mighty concrete columns supporting the tracks above, squat and spreading, where you can feel the weight of the construction. There are the beginnings of the undulations, in the platform canopies, that made the Melbourne station memorable. There is potential for fascination in the glimpses you get of motion, human and mechanical, if you look up and sideways at the multiple layers of tracks, bridges and escalators.

But Grimshaw also designed the revamp of the station in Newport, Wales, a project lucky not to win the 2011 Carbuncle Cup, which achieved a striking form visible only to seagulls (and for some reason uterine) at the expense of happy relationships at the level of the human eye. Some of the Newport station’s spirit has turned up at London Bridge, and it turns what could be a great project into one that is good in parts. Curves don’t quite sweep as they might and edges you might want sharp are blunted. Too much that might align doesn’t, and materials and shapes too often clash.

The platform canopies struggle for their elegance, both against the cladding panels that muffle their structure and against redundant steel frames inserted in the air space, which were bizarrely insisted on by a local planning officer. An old war memorial, reinstated from the old station, finely lettered in cast bronze, finds itself framed with ill-fitting cladding panels, held in place with a firing squad’s scatter of black plastic fixings.

The concourse has the ‘potential for fascination in the glimpses you get of motion, human and mechanical,

if you look up and sideways’. Photograph: Andy Hall/The Observer

At this point, credit for the work has to be spread more fairly. This is not the responsibility just of Grimshaw, but of a design team that also includes the management, design and engineering consultancy Arcadis WSP/Parsons Brinckerhoff, which is in turn employed by the contractor Costain. It is made under a design and build contract, by which the exigencies of construction usually trump an architect’s pleas for more considered detail.

Nor, even, does the buck stop with Costain, but with a wider culture of delivery. It would take a small cost in thought, possibly zero, to have made the new London Bridge exceptional. Conceivably it could have saved money – there’s probably a cheaper but better way, for example, of honouring Victorian heritage than the parody of the 2D arches. But the corporate systems for projects such as this can’t find room for this kind of consideration.

Demolition is imminent for the station’s 1893 South Eastern Railways offices, left, which will be replaced by ‘an expanse of plaza’. Photograph: Alamy

On the north side of the station, the final phase is under way. Here, shrouds have gone up round the South Eastern Railways offices of 1893, in preparation for demolition. In its craft and detail, in its ability to make different materials and techniques into a cohesive whole, this building has – the present tense can be used, just, for now – the qualities hard to find in the rest of the station. It also makes use of its tricky triangular site to make what has been called Britain’s version of New York’s Flatiron building, which tapers engagingly to a narrow end.

It is going in order to make way for an expanse of plaza, to go with other expanses of plaza around the project. Although the architects insist that this was unavoidable, it’s hard not to wonder whether a bit more thought would have made it possible to open up the bottom levels of the old offices and make them into a public arcade. But it is this kind of thought that the project lacks.