I haven't been on the Zig Zag yet but might at some time in the near future. Unfortunately there hasn't been much time to do that sort of thing.

There is indeed five modes of public transport across the Sydney basin and out to Lithgow, Newcastle and Kiama - all covered by the Opal Card ticket system.

Heavy rail - the big network with double-deck trains operated by drivers and guards.
Metro rail - single-deck trains with no driver or guard on board.
Light rail - three lines from the CBD out to Dulwich Hill, Randwick and Kingsford. A fourth line has been built in Newcastle and a fifth is under construction in Parramatta. Three different models of trams are used, depending on the line.
Buses - the blue Mercedes is still out there.
Ferries - our famous harbour ferries still ply the world's largest natural harbour.

I didn't realise there were going to be different trains on the yet to be finished metro lines. The Alstom trains on the north-west line seem fine. Why would they bother going with different models and numbers of cars per train, given that one day all the lines under construction will one day be joined together on the outskirts of the metropolitan area... Bringing AC into the mix is daft too, given that the current system runs 1,500VDC on all electrified lines in NSW. To me, this is no different to each state having its own rail gauge like back in the 1800s. Fixing this later when all lines do get joined into one network (probably about 20-30 years away) will run into some money.

The plan as I understand it is to eventually convert all the lines to driverless running.

DC overheads and any equipment connected are exceptionally vulnerable to lightning hits. You only have to notice the system outages that happen when there's a decent electrical storm.

The only reason we have 1500VDC is the system was implemented in the 1920s. The idea being that this was the voltage the traction motors at the time used. It kept the large and heavy conversion equipment trackside, and the trains themselves simple.

Technology moves on. In over 100 years, I guess you must expect this!

My father joined the Electricity Commission as was in the fifties and was sent around various Sydney power stations for training before going to serve in country hydro stations. He remembered mercury arc rectifiers used to feed the city DC network of the time. Might have fed the railways too. These were massive glass constructions containing a pool of mercury and probably would have been too fragile to put on a train, let alone too bulky. Technology has eliminated these too.

I doubt the heavy rail network will ever be converted to driverless - it is too complex and any system fault may see trains derailing or crashing in to each other. There would never be a return on the capital invested in making a conversion anyway. This is why the metro network is being kept structurally seperate and simply co-mingling at several strategic points such as Epping, Chatswood, Central and Parramatta. One day, with the current expansion plan, Schofields, Bankstown and Liverpool will be served by both heavy and metro. One line that may get converted will be the South-West rail link, comprising Edmondson Park and Leppington, which will connect a new metro line between Tallawong and Glenfield via this line.

As for lightning, it can blow anything up. It even strikes deep in the ground to hit phone lines and zap subscribers if they are on a call during a storm. QLD uses AC for its electric trains as they came along relatively late in the peace. A comparison is probably in order. That said, the tech on all new trains is much the same. The only difference is the voltage between the pantograph and the traction system and how it is managed, converted and/or inverted.

A little bit of history here. At the time of their introduction Sydney 1500V trains were world leaders.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjEr7-wypv_AhXtTGwGHXYICJoQFnoECBAQAQ&url=http%3A%2F%2Fschoolpa.com.au%2F~paul%2FHET%2FCoolpage%2Fblurb.html&usg=AOvVaw1-StWrG8SKUU62TiA3h_Sg

"Before 1926, only 750 volt traction motors had been built. The
need for only two motors meant that Sydney's cars required very
high powered 1500 volt motors. At the time they were introduced,
Sydney's electric trains featured the most modern and advanced
electrical traction technology anywhere in the world, the very
first motors in history to utilise 1500 volts directly across a
single motor. These motors were amongst the first to employ
manufacturing methods now considered "essential" in modern
electrical design. "

The first 50 cars, built by Leeds Forge in Leeds, England, had two 750V motors in series on each power bogie as a stop-gap measure. When Clyde Engineering (now called Downer EDI) started building almost identical cars here they switched to 1,500V motors. Even back in the early years, all cars, no matter who made them, were all interchangeable and would work with each other and backwards compatiblity was maintained right through to the second-edition double-deck cars, AKA: S-sets.

The manufacturers included the factory in Leeds, Clyde Engineering at Clyde, Commonwealth Engineering at Granville, Walsh Island Dockard at Newcastle and Tulloch at Rhodes. I think the Comeng cars were the ones known as Sputniks because they were built at the same time the Russians launched their first satellite by the same name. The coachwork of these cars were spot-welded rather than riveted and were the only ones that had doors operated by the guard.

As a kid I was always mesmerised by the huge contactors underneath the train throwing sparks when the driver would back off the juice. Fortinately the contactors had good spark quenchers built in. Bleed resistors also featured to control speed.