Perth's Horseshoe Bridge — named for its unusual shape — continues to be a vital link for cross-city traffic, even though the railway line it once spanned is now underground.

"This is one of Perth's icons," Richard Offen, executive director of Heritage Perth, told 720 ABC Perth.

"Everyone knows of the Horseshoe Bridge because of its unusual shape."

The bridge connects the city and the Northbridge section of William Street.

Its birth came during Western Australia's first mining boom — the gold rushes of the 1890s.

Aerial view of Perth featuring the Horseshoe Bridge in 1922. ( Supplied: State Library of Western Australia )

"When the Guildford to Fremantle railway line opened in 1880 there was actually a gated level crossing here at William Street," Mr Offen said.

"Each time a train came through a gatekeeper would come out and [open and] shut the gates — there were none of these modern boom gates.

"As the population grew during the gold rush, the frequency of trains grew and so did the road traffic.

"The result was gridlock in Perth."

A footbridge for pedestrians was built in 1896.

Controversy arises after bridge opening

By 1903, the State Government realised it also needed to build a traffic bridge across the railway line — but there was a hitch.

"Because the buildings on William Street were quite close to the railway line on both sides, there was going to be a problem building a conventional straight bridge," Mr Offen said.

It was Robert Howard, a draftsman in the Public Works Department, who hit on the idea of bending the bridge around the buildings.

"The bridge opened in 1904," Mr Offen said.

"But it was immediately surrounded by controversy, because there were plans to demolish the wooden footbridge and an awful lot of people started complaining about it.

"Somebody worked out that they would have to travel an extra mile each week because the horseshoe bridge was longer."

No more traffic jams ... the Wellington and William streets intersection in 1924. ( Supplied: State Library of Western Australia )

The Daily News took up the issue, chastising the pedestrian opponents, in an editorial on July 8, 1904:

The structure had been built practically on the lines desired by the people, and now objection was being raised to the unsightly wooden footbridge which was never intended to be a permanent structure. It was a menace to the safety of the railways, and yet after all that had been done to meet the citizens' desires some people wanted two bridges instead of one.

"As usual, things calmed down and people forgot about that and realised that there were no more traffic jams," Mr Offen said.

The bridge was in use until 2009 when it closed for seven months for repairs.

The city-side entrance to the Horseshoe Bridge. ( 720 ABC Perth: Emma Wynne )

In 2010, work began on the $360 million Perth City Link, which involved sinking the railway line under the Horseshoe Bridge and opening up 13 hectares of land for development between Northbridge and the CBD.

Trains began running underground on July 18, 2013.

The space between the arms of the Horseshoe Bridge is now being developed into a public precinct called Yagan Square.

And although the bridge no longer spans the railway line, it is still in use and now listed on the state heritage register.

"There aren't many bridges of this shape in the world and it has played a unique part in Perth's transport history," Mr Offen said.

"They have kept the gorgeous pedestal lamps which light the bridge and it is now going to be a wonderful backdrop for the new Yagan Square when that opens."