Engineers have installed the first line of track through the kilometre-long structure.
Features of the project included removing over 5,000 tonnes of concrete and rock, creating a new 945 metre concrete base and installing 820 metres of new slab-track units along with 2,000 metres of new rail.
The renewal scheme is being delivered by workers on the Edinburgh Glasgow Improvement Programme (EGIP) to electrify the main Edinburgh-Glasgow line.
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Work is now underway on laying the second line of new slab-track through the tunnel and the high level station is on schedule to reopen on 08 August.
David Dickson, ScotRail Alliance infrastructure director, said: "We have now passed the half-way point in this vital project and our engineers have completed a huge amount of work as they stay on course to reopen the tunnel for passengers on August 8.
"The renewal of the tunnel track, and the installation of overhead power equipment, will allow the introduction of faster, longer and greener trains on the main Edinburgh-Glasgow line – delivering thousands of extra seats, shorter journey times and improved accessibility for customers."
The redeveloped Glasgow Queen Street station is due to be completed in 2019.
(LM)
