Network Rail announced plans to lengthen platforms five and six at the railway station in order to accommodate the longer inter-city trains due to be used on the East Coast Main Line and the predicted doubling in passenger numbers at the capital's busiest station by 2024. The new trains are due to be rolled out from 2018 as part of the new East Coast Main Line franchise, reducing journey times between Edinburgh and London by 15 minutes and boosting capacity on the key route.
Neither platform is currently in public use.
Platform 12 will also be extended to pave the way for the introduction of the eight-carriage trains which will be used on services to the west once an electrification project is completed.
The investment at Waverley is the latest in a series of improvements at the station totalling more than £300m over the last 10 years, including renewing the roof and introducing lift and escalator access to Princes Street.
The work is expected to start in October and is scheduled for completion by March 2016.
It is jointly funded by the Scottish and Westminster Governments.