There is already a tunnel in place at the 600m (underground) so will only require some development on the access to and from it to make it work. Mainly maintenance vehicles, track vehicles etc. use it now.
Re existing car parking - my understanding was this was to be come leasehold land with a shopping precinct, hotel and apartments similar to Alex Park however it never came to fruition due to the financial crisis a few years ago - maybe it is back on the cards. Access and egress had been council approved for this about 8-10 years ago (I think) but no idea what is happening now.