The first of the 10,000 tonnes lying under the twin tracks has just been excavated as part of the five-month project.
The £70 million scheme is also the most expensive of its type over that timescale, involving 450 engineers working round the clock in three shifts.
Programme director Rodger Querns said it remained on schedule to be completed on 8 August.
Track in the mile-long tunnel which is up to 40 years old will be replaced, and lowered to provide space to power new electric trains.
Platforms on the main concourse are being lengthened and realigned in the first part of a major revamp of the station.
Trains are being diverted to below-ground platforms or Glasgow Central.