I would like Sydney Metro Western Sydney Airport to use 4-car Bombardier MOVIA trains instead of 6- to 8-car Alstom Metropolis trains, and electrify from bottom-contact type third rail (750V DC), NOT from overhead wires (1.5kV DC). Advantages of third rail include:
The cost of a third rail is lower than that of overhead wires, and it is less affected by natural disasters.
The third rail is more suitable for installation in tunnels with smaller clearance than overhead wires.
The subway third rail does not need to be erected under tension and is not prone to breakage accidents. Even in the event of a failure, the scope is smaller and the reliability is higher, thus reducing the maintenance workload.
The installation position of the third rail is relatively low, there are no obvious tall parts, the urban landscape is good, and it is easier to take protective measures against electromagnetic pollution.
The preferred voltage of a third rail is half of that of the overhead wires.
I know the CBD and South East LRT also uses third rail between Circular Quay and Town Hall, but it's not using a conventional third rail; it's using Alstom's APS which unlike conventional third rails don't pose any danger to people or animals. However, for subways it's much easier to use a third rail for electrification.