>>1325336>A big city (1-2 million inhabitants) ideally should have 3 layers of urban rail:>1: cross city mass rapid transit (metro) express lines>2: regular mass rapid transit with more stops>3: tram with many stops for maximum coverage and to connect quieter areas
First off, 1-2 million is not a big city. It's a medium sized city, big is about 2.5+ million.
That said, I don't think small and medium necessarily need two layers of heavy rail. In Europe it's common, but those are also usually very small-profile metros, like >>1325337
says the kinds of systems that nowadays would almost be considered pre-metro or even LRT.
Just one heavy rail system, a commuter type (RER or S-Bahn type) system which offers fast travel times with semi-distant stations in the city, possibly underground but not if not necessary, and has commuter range outside it, possibly using preexisting rail lines, would do perfectly fine for cities up to 2 million. This obviously means that this system would have more density than it usually has in cities which already have a normal, metro-type heavy rail.
Add to that LRT(/tram) for local urban transport.
What I'm saying is that if the same effort is put into just that one heavy rail system plus LRT, it's going to be much more fruitful than if the same effort goes into commuter, metro and LRT.
Many German cities do this kinda right, although some fell for the Stadtbahn meme, especially in the west. The big east german cities are breddy gud, like Leipzig and Dresden. Large tram networks (usually getting upgraded to some sort of ROW or segregation, if not present already), and S-Bahn. No metro.
Essentially, being former GDR cities, they didn't get anywhere close to the sort of investment that west german cities got, and yet with much less investment they offer a very good quality public transit. Which goes to show that it's a more efficient concept.