This is a video I made of myself in downtown and central Portland:
Not all of it is transit-related, but significant parts of it are. (And even the parts that aren't, are related indirectly)
There are two things that are probably the most important. The first is the Portland Streetcar, which opened in 2001. When I lived in Portland, up until 2009, it wasn't a very useful thing, because it mostly just went around the core downtown area, and it mostly carried shoppers. It was mostly a convenience, and not even that, because usually the time to wait for it was longer than just walking, at least for me. (I was in my 20s and had a lot of energy
). So, like a lot of Streetcar systems, it seemed to be a cute way to attract suburbanites to a downtown area. But over the years, it expanded, and it now is long enough that it makes visiting certain places a lot faster. And I just checked---in 2019, it carried 5 million people a year, which makes it about half as busy as, say, Sacramento's "Real" Light Rail system. (Even compared to Portland's MAX system, it is considerable, carrying about 1/8th as many passengers). So I was pleasantly surprised to see how useful the Streetcar could be.
Towards the end of the video, I show the Steel Bridge, which is the double-lift bridge that carries both the MAX Light Rail on the top deck, and heavy rail (including the Coast Starlight and Cascades) on the bottom deck. When the MAX was first built, there was only one line. Now, all of those lines go over this bridge. So this bridge is one of the limiting factors for how many trains they can run through downtown.
Both of these points are related, because it shows how transportation infrastructure can sometimes scale in ways that are not expected. When the MAX opened in 1986, and even when the Streetcar opened in 2001, Portland was quite a different place than it is now, and so sometimes transportation plans have had to adapt to the city changing.