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The best transfer station I’ve seen was when visiting Taipei.

They used large diameter boring machines for some of the lines, which allowed for putting opposite directions on top of each other, rather than the standard next to each other configuration.

So at a junction, you would have four tracks inside of one tube, and the other line be right across when the train stops. Now they sync the two different lines to always arrive simultaneously. Thus a transfer is you go out of one train, and right there, across is your connecting train.

I’m not sure how much the whole thing cost to implement and maintain, but I do think it was all worth it. It allowed them to have really accurate and fast travel times across all the lines connected this way.

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London's Victoria line was built for "cross-platform interchange" like that (often by having its lines arrive on the "outside" of another line, so that both tracks were alongside same-direction tracks). Nowadays it's something they generally avoid, on the grounds that it reduces circulation space - with the system running close to capacity, they prefer to have everyone walk further so that the crowds spread out a bit.

It works if you have very wide platforms. The general issue is that London doesn’t build large platforms suitable for cross platform interchange.

London also uses this crowding excuse to deliberately not build transfers, like the new Northern Line extension with the Victoria Line.

Mong Kok in Hong Kong is the first I’ve seen these type of cross-platform interchange: https://en.wikipedia.org/wiki/Mong_Kok_station

That would be great if feasible, but I think most transfer stations have lines crossing each other at near 90° for obvious reasons, which would make this impossible.

You’re right, it wouldn’t be possible.

I’m in Taipei now, not every intersection looks like this. But I remember seeing it at Chiang-Kai Shek Memorial Hall station (https://en.wikipedia.org/wiki/Chiang_Kai-shek_Memorial_Hall_...). On the Taipei MRT map you’ll see that both the red and green lines make a change in their trajectory here, allowing them to go parallel.

I think the technique the OP described is “necessary, but not sufficient” for these types of transfers.

If that's the only issue you could have both lines turn 45° before and after the station so that they're parallel for a short stretch.

Well, that's only going to work if there's enough underground space for railways to turn 45° before entering the station. Judging from what I've seen, most "important" transfer stations are in an urban district where railroads follow existing roads, so curving them would send the lines underneath existing buildings, making it more expensive and also slower (trains don't like to turn, after all).

Following roads is generally specific to a few early metro systems, I don't think it's a common technique in general.

I think most of these stations are on networks that had a coherent planning that was followed through from the beginning - like in Munich, where the U-Bahn network started to be built in the 1960s (late compared to Berlin and Hamburg) and was continuously under construction, mostly following the original plan, until 2010. Munich has two of these cross-platform transfer stations, Scheidplatz and Innsbrucker Ring. Both of them were built with 4 tracks from the beginning and had to wait several years (or even decades) until the cross-platform transfer concept was actually fully used with two lines coming from different directions and also continuing in different directions. There are two more four-track stations (Münchner Freiheit and Olympiazentrum) which will probably never be used as cross-platform transfer stations - I think at Olympiazentrum the additional tracks were intended to increase capacity for large events (football matches, concerts etc.).

This is kind of hybrid I guess, cross-platform on one level and a side platform on the other.

https://en.wikipedia.org/wiki/19th_Street_Oakland_station

That's how it works in Lille Flandre, which is depicted on this website. In Lille, in the North of France.

They're just all on the same level though.

I can understand how this works under ideal conditions, but how does this system handle delays?

Well I guess it degrades to normal - go out of the train and wait for next train to come.

The remarkable thing is when working as intended it removes the “wait” bit speeding things considerably.

Locals probably have better info but all the time I passed through that station (about 8 in the span of a few days) trains were in sync.

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