London has terminus stations - the platform the train arrives on is where the train stops, and then starts on its return journey. Trains to your destination are therefore limited by signalling and traffic, typically a 10-15 minute turn around time. On average, in London during rush hour, you have a train to your home station every 20 minutes.
Perhaps in the US all major stations are through stations, but in the UK, especially London, we just don't have that, and mezzanines are at the very minimum, a safety requirement, because the platform your train leaves from, has other passengers waiting for other trains before your one.
Platforms are not large, and trains are very, very busy, and frankly, if you've only ever caught a train in the US you are in for a huge surprise commuting in the UK.
If the US ever adopted trains like the rest of the world has, the best time for building mezzanines, and huge stations is "yesterday".
To expand capacity beyond that would require reengineering the tunnels themselves. They go as fast as they can turn around and vent air as it is. You'd have to bore parallel tunnels to increase capacity, which would just mean adding more platforms and still wouldn't require a mezzanine.
This in a city of around 2 million inhabitants for the whole metropolitan area, which is also served by busses, trams, local trains, ferries and so on.
Great thinking ahead, except plans changed and when (decades later) they eventually decided to build the Eastern Suburbs line, they chose a completely different alignment. So they just had these stub tunnels sitting underground, disused. During World War II they were converted to being an air raid shelter. They've seen some other random uses over the years, such as being used to film TV shows, part of them is used as a maintenance train siding, etc. Now the government is seeking proposals for the private sector to redevelop them into something useful.
When the Eastern Suburbs line was finally built on a different alignment in the 1970s, they did something similar – it has two underground platforms at Central Station, but they actually built four underground platforms. Two of them active, the other two with stub tunnels not going anywhere. Once again, this was to support another planned railway line, the Southern Suburbs line, which has never built either. For many years, the never-used platforms were used as archival document storage for the government rail authority. Now, Sydney is building a new metro line. The Metro line is not going to use these platforms, it is using newly constructed underground platforms. But these never-used platforms are going to be converted into utility rooms for the Metro line.
Similarly, at Redfern station, there is a big pit in the ground which holds half-constructed platforms for the Southern Suburbs line. I doubt those platforms will ever be used for that purpose.
So anyway, planning ahead is a great idea, but often it just turns into a waste.
Yeah, usually by totally rebuilding the station. Easier said than done when you already have a skyscraper built over top of you. NYC is trying to revamp a lot of its current stations that see magnitudes of traffic more than they were designed to on a daily basis, and that's a massive undertaking.
That's good for track maintenance too.
For 2.5 billion dollars, I bet they could have extended the system, not built some insane labyrinth 150 feet underground and improved signaling to run 30 trains per hour and maybe even built that 2nd station they had to cut due to cost.
The new signaling for the 7 will already get them to 30 trains an hour, and that will not be enough in 30 years, much less 100.
For example, Strong Towns [1] seems to champion the whole "the suburbs are unsustainable" perspective, but never the "public works projects in HCOL metros are too expensive for the resulting density" thesis. Market forces will likely work it out in this case though (people leaving HCOL locations for lower density locations where per dollar quality of life is higher, with major cities having to do more with less, or do less with less [tax revenue] [2]).
[1] https://www.strongtowns.org/the-growth-ponzi-scheme
[2] https://www.nytimes.com/2020/08/27/nyregion/mta-virus-budget...
Everyone in NYC hates it and we are currently building an extension at the old post office, as it is not fit for usage anymore.
The old one (the one that got demolished in the 60s) was miles ahead in both space and aesthetics. Also, almost nobody hates the Grand Central Station, even though it gets overran with tourists as it is a beautiful peace of work.
1. Amtrak insists on trying to do an airplane-like boarding situation for trains, rather than the traditional practice of telling people what track their train will be on several minutes in advance and letting them wait on the platform. This means you have boarding lines and expectant crowds in places that aren't designed to handle that kind of capacity.
2. Each of the services that call at Penn Station has its own segregated platforms, mezzanine, and ticketing areas, and the connectivity between these is poor and confusing. Although, this is probably driven in large part by...
3. Madison Square Garden was built on top of Penn Station, and a substantial portion of the above-ground infrastructure is dedicated to supporting that tenant rather than the transportation infrastructure.
Penn Station used to have a more normal major-station layout when it was still part of the Pennsylvania Railroad, but that was ruined by the building of Madison Square Garden on top of it.
Penn Station's major issue was that when it was designed and built, intercity traffic was a big deal and commuter traffic was not, and it was designed accordingly. Today the central platforms Amtrak uses are massively oversized and the commuter-rail only platforms to the side take 3-5 full minutes to clear a train.