Lucky old London. No sooner does someone come up with a smart idea, then everyone falls over themselves to fund it.

One of the latest is a garden bridge, to cross the Thames somewhere near the South Bank.

There’s a designer, there’s a celebrity proponent in the shape of Joanna Lumley and now there’s a planning proposal.

The Mayor of London (if he’s still there) has offered £30 million towards it and the government (if they’re still there) a further £30 million.

That should be enough to make a very nice bridge.

The idea is not entirely new, of course, though it’s a perfectly sound one. There’s a similar “garden in the sky” in New York, known as the High Line, running for a mile along the former Central Railroad on the West Side of Manhattan.

So how about a Birmingham garden in the air? I don’t mean one of those vertical shrubberies that have recently begun to colonise office walls but a genuine garden with a view.

Indeed, you could argue that the city already has one. There’s a stretch of elevated railway which snakes across Digbeth, built for the Great Western, but never used. Nature began to re-claim it almost as soon as it was constructed in the 1840s.

Walk along Fazeley Street and you get a fleeting glimpse of it from below, a wildlife jungle 40 feet in the air. I’ve come close to walking along it, but there’s a forbidding sign, and threats of prosecution, health and safety warnings etc. I always lose my courage in the face of a conviction.

The best view from above – short of commandeering a police helicopter – is on Google Maps, which shows a thin green line that starts around Adderley Street and then peters out close to Montague Street.

The only snag with it is that the bridges over the roads were demolished.

Unless you limit the park to Olympic long-jumpers, you would have to replace those.

Plenty of abandoned railway lines have been turned into green walkways – one in Harborne and another in Wolverhampton – but none with quite so spectacular an elevation.

So let’s go ahead and do it, Now, where’s the money?

* Dr Chris Upton has his head in the clouds at Newman University Birmingham