As someone who used to live in New South Wales and commute by train, I can perhaps add a little background to Michael Darby's piece that Patrick has linked to below.

Sydney has a suburban rail system owned and run by the state government. This system covers some parts of the city quite well, but other parts of the city not at all. There are some parts of the city from which it is possible to commute by train, other parts from which it is possible to commute by ferry on the harbour, and other parts from which the only way to commute by public transport is by bus. (These are some of the poshest parts of the city. I personally find a bus commute intolerable, and wouldn't live in these parts of the city unless I could commute by car). There is no separate underground system in Sydney. Mainline services typically enter tunnels as they approach the city centre and go under the city centre (where they make a few stops) before coming up again and returning to the suburbs. This means that underground trains services use much wider tunnels than is the case in London, and possibly their extent is limited because of this. (The system is perhaps similar to what Paris would be like if you had the RER and no metro).

One interesting feature of the Sydney train system is that virtually all trains are double deck. You enter the train, and then go either up or down a flight of steps. If you go down, the floor of the carriage is well below platform level, and the bottom of the carriage windows is at about platform level. If you go up, you are higher than is common in trains in other countries. The overall height of the carriages is not any higher than you would see in a British mainline train, which means that getting on a suburban train in Sydney feels a little like getting on an underground train in London: there isn't a lot of headroom and you have a relatively large number of people in an enclosed space.

The entire Sydney suburban system was built by the government. One consequence of this is that it has a uniform loading gauge, and a uniform system of electrification (although this is a 1500V DC transmitted by overhead wires, which is 1920s technology and is not a system that anyone building a new railway system would use). As a consequence, the same rolling stock can be used throughout the system, and trains that are on one line this week could end up on any other line next week.

As for procurement of rolling stock, the system isn't big enough to support a continuous process of retirement of old trains and building of new ones. What happens is that every fifteen to twenty years a government realises that new trains are need, and a large order of some new generation of trains is made. These come off the production line for a few years until the government decides it has enough trains and production of that generation of trains stops. Every generation of trains produced in the last 30+ years has been double deck and the all have had approximately the same dimensions, but quite a bit of trouble has been gone to to ensure that each generation looks different on the outside. However, this state of affairs in which no trains are made for ten years, then a lot for five years, and then none for the next ten years etc does not lead to efficiency. Train building companies have to ramp up production, then ramp down production a few years later, and then the whole process repeats itself.

All this needs to be borne in mind when reading Michael Darby's piece. Another key fact is that a state election is due in New South Wales in a few weeks. A new generation of trains (the so called "millennium train") has just gone into production. They are suffering all the usual teething problems that occur when you open a new production line. The state government is making a big show of having the new generation of trains running on the rail network to try and score political points. The opposition is blaming the government for all the teething problems in order to try to score political points in the opposite direction. It is all pretty boring politics. In my opinion the teething problems are not really the point. What is the point is the age of the trains on the system. And the only relevant point in this regard is the size of the order for trains. This is actually too small, and some of the old non air conditioned trains will still be on the network for a long time.

And in Sydney, the question of whether the trains are air conditioned is extremely important. In summer, it can be over 35 degrees and very humid even when commuting at 8.30 in the morning. A crowded, non air conditioned train can be horrible in those circumstances, particularly if it is double decked and there isn't much headroom. What happens is that you wait on the platform waiting for your train. If the train you see approaching is one of the newer generations that is air conditioned, you feel relief. If you see a forty year old trainset coming toward you, you inwardly groan and you suffer on your way to work.

And as for trains being overcrowded, this is just political posturing. There is really not much that can be done about this. The sections of line through the centre of the city are at capacity during peak hours, so the only way to make trains less crowded would be to build another line through the city. While there are plans on the drawing board to do this, and there is an underground path through the city reserved for a new line, this is a long way down the government's list of transport priorities. (The present Labor government has a good record of building roads, but its rail projects are generally a mess).