They can't even manage to properly run the toy train system they've currently got (it's closed yet again for a week for "urgent repairs"), let alone keep trying to add more to it. They would be better off disbanding Auckland's trains and converting the rail corridors to bus lanes instead ... which would also mean less need for many of the bus lanes on the busy normal roads and allow more easily flowing car traffic.
The network is run by Kiwirail, not Auckland Transport, and its state is a result of many years of underfunding and lack of parity with road based funding priorities. Busways, including underground bus interchanges, was investigated as part of the recent CRL funding requirements and deemed to not be the most effective route. Converting it now would just cause even more problems with respect to pollution until a fully green ev fleet is established and then the cost to rip out would be astronomical compared to just fixing it.