So. These bridges. Chicago is not just home to some of the world’s earliest skyscrapers, it is also a city of bridges. There are five different types of movable bridges and 43 are still operable. The first was timber-built in 1834, similar in design to a medieval drawbridge. The first swing bridge was built in 1856. It was too narrow and eventually collapsed under the weight of a herd of cattle being driven over it. In the mid-1890s vertical lift bridges were developed, pulled up and down from huge counter weights held in towers on one or both sides of it.

Down the main drag of the Chicago River, through the middle of the city, there are 10 bridges, mostly trunnion bascule bridges, the first of which opened in 1907. The leaves of the bridge which span the river are suspended on axles, trunnions, with massive concrete counterweights located below in the river bed. Single leaf bascule bridges were built when the river was narrower and mostly carry subway tracks. Double-leaf bridges stretch from each bank where the river was wider and meet in the middle. All these bridges are opened together at specific times in the spring and autumn to allow high-masted vessels to move up and down the river, to and from winter moorings.

Bridge-tender houses stand sentinel alongside each and reflect different architectural styles.

Some of the bridges in downtown Chicago are double-decker with vehicular traffic using the bottom level and subway trains using the upper level to link up with the overhead gantries at either end. Life down here is pretty medieval. Dark, dimly-lit corridors bore out over the water, protected by huge, riveted balustrades, all covered in anti-rust paint, like an earthy sunset. As the lights change a snarling collection of fiery vehicles charge across, headlights blazing in anger and engines roaring in the echoing darkness.

On the upper layer, silver silk-worms creep out from their holes in the concrete and glassy jungle and over, always exposed to the elements and reflecting the weather on their matted surfaces.

From the bridges raised pillars support the subway gantry above, creating more corridors of flickering sunlight and shadow on the streets below.

The noise is tremendous. There is nothing peaceful about Chicago’s streets. Not only do you get the normal city noise of vehicles – horns, engines, tyres. But as trains approach above, the tracks vibrate, metal hums, the timbers shudder on the metal framework. The clatter and clack of wheels, the squeal of brakes, the complaining of carriage metal on bumpers, the whirl of engines, all intensify as the train passes overhea. It recedes as it disappears into the distance, leaving the pedestrian with more normal street noises until another approaches.