Long-abandoned underground train tunnels and platforms built almost a century ago underneath Sydney could soon house bars, bustling shops and cool cultural spaces.

The NSW government has asked companies from around the world to present how they'd open businesses in the tunnels and on the unused platforms at St James Station under Hyde Park.

The infrastructure, designed by Sydney Harbour Bridge engineer John Bradfield, was constructed in the 1920s but the two rail lines it was meant to serve were never built.

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The disused tunnels and platforms have hosted a mushroom farm in the 1930s, operated as a siding for trains until the late 1980s and featured in films including the Matrix - but they've never been accessible to thousands of people on a daily basis as originally envisaged.

Sydney Trains chief executive Howard Collins says it's not right the structurally sound 6000 square metres of historic space has only been open to a lucky few.

Mr Collins, who previously ran London's Underground, on Monday said many global cities transformed unused tunnels into bars and other tourism ventures.

"Why should it be just railway employees and a few special guests who see these sites?" he told reporters on one of the unused platforms.

"We are not the specialists in this area so we are asking the global market to express their interest and say what we can do with this space which will really be an attraction for people to sit, drink, dine and explore. It is a blank canvas."

Access to the tunnels is currently through a door on St James Station's operational platform so would need to be modified for public access.

There'd also need to be a clean-up to rid the tunnels of leaky pipes, piles of gravel, discarded pipes and sheets of metal.

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"We transformed the Central Station precinct - we could do the same here," Mr Collins said.

Transport Minister Andrew Constance envisages small bars and restaurants doing business within a few years but is open to any possibility.

Expressions of interest close on November 6 with construction potentially starting next October.