The installation consists of two 2m diameter tubes mounted on posts (perhaps) 5m above the ground. At each end is a terminus with a store of ready-to-use cars and an air-lock.
Bookings are probably not necessary as cars will depart every 20 seconds; just pay and go.
In Sydney, the car is filled with passengers and their luggage and then sealed. The car is then moved into an air lock mounted to the side of the tube. Once sealed, the air lock is evacuated of all air and when ready, the separation is opened and the car is inserted into the main tube.
A linear motor (also known as a MagLev) accelerates the car at 1G, reaching full speed in around 650 seconds (a little under 11 minutes), by which time it has already travelled over 600km.
Around 600km from Perth, the MagLev engages the car as a generator and recovers almost all of the energy as it decelerates (also at 1G) to rest at the terminus. The car is shunted into an air lock, which is filled with air immediately after the tube door is closed. Soon after, the car emerges into daylight and the passengers disembark. A total of 40 minutes travel time.
Of course a simple point-to-point service is of little use; we need to establish a full mesh service. Hint - Melbournites really don't want to travel to Sydney in order to get to Perth. This means there must be a means of merging and diverging tubes.
In addition, there are hills - so a means of accelerating the car up and then recovering the energy on the down is required. Nothing difficult there.
Estimates suggest that a fully operating system will cost around one tenth of an equivalent highway (not including fuel and other costs) and around one quarter of a high speed rail system. And of course ETT has considerably lower maintenance costs than either and due to deceleration energy recovery is exceedingly cheap to run.
Are there disadvantages? Of course there are.