So for the last several years there's been construction outside the east exit of Shibuya station. You notice it particularly if you bicycle through there because there are metal plates on the ground with light metal washers in the corners, and when you ride over them it makes a very distinctive noise.
After awhile I heard (separately) about the fact that they're going to join up the Tokyu Toyoko line and the Fukutoshin line to run through at Shibuya (this means that Toyoko line trains won't terminate at Shibuya, but will just continue, becoming Fukutoshin line trains. They use this system a lot in Tokyo and it is very efficient). I eventually put two and two together and realized this must be what the construction outside the east entrance is about; they're building the new part of the Toyoko line to meet up with Fukutoshin line.
But then I realized the Fukutoshin line is deep; it has to be below the Den-en-Toshi/Hanzomon line, which is perpindicular to it under the ground at Shibuya. And that's when I realized the entire east exit of Shibuya station must currently be suspended about 60 feet in the air. Those metal plates are covering the top of a steel latticework that goes all the way down to the level of the Fukutoshin line, so literally all of that traffic is currently driving around on a causeway 60 feet in the air.
Sure enough, if you poke around at the edges of the construction, you can find some cracks that verify this is indeed what's going on! It made me a lot more queasy about riding through there to think that the entire east entrance including the road I am riding on is 20 meters above the actual ground.
Last week I moved out to Shimokitazawa, and I live quite close to the Odakyu line near Setagayadaita station. By reading the signs out here, I've realized they're also doing a big construction project on the Odakyu line as well. After staring at the signs for awhile, this project is even more amazing: they're taking the entire Odakyu line and burying it under the current, fully operating train line from Higashikitazawa station to Setagayadaita station. And they're not just burying one train line down there, like in Shibuya; no, for most of the length they're creating two train lines, one on top of the other, under the current line; an upper line for local trains and a lower line for express trains.
At Setagayadaita station, both of the new buried lines have to dive even deeper underground to get under the underpass of Kannana-dori. So just a minute and a half from my house, when I walk across the grade crossing the railroad currently has, I'm actually walking across a 27-meter high trench with three levels of train tracks in it. All this while the line is in heavy operation (the Odakyu line is a major commuting line). At this intersection, it's really easy to look down in and between the girders you can catch a glimpse of the fact that yes, you are close to 100' above the ground.
Check out the page here, in Japanese to see diagrams of how complex this track-burying process is.
Now, why would they go to all this trouble? The reasons cited on the linked page (oh, we can improve train service by running more expresses; oh, having no grade crossings makes things safer and more convenient for local residents) are, as far as they go, true. But they can't possibly justify the huge expense of these projects (the track burying construction project on the Odakyu line I'm describing was started in 2004). What can explain the willingness to spend the crazy amounts of money needed to bury these rail lines? After all, Odakyu is a private company.
The answer, of course, is real estate. By burying the tracks, Odakyu will come into possession of a huge amount of real estate, much of it conveniently located near major railroad stations. So despite the mind-boggling amount of investment, afterwards they'll own the most conveniently located, eminently developable land in Shimokitazawa.
Going back to the original topic, in Shibuya the motivation for the Tokyu Corporation is even more blatant. The current Shibuya Toyoko line station is huge: it's a four-platform terminal station that takes up a ton of space. While the through-running system undeniably helps Tokyo trains be admirably efficient, it's more likely that the real reason Tokyu is motivated to bury the Toyoko line is because when they're done tearing down the current station at Shibuya, they will have possession of dozens of acres of land directly adjacent to Shibuya station. It's almost incalculable what the development rights to that land will be worth.