Cool, yes I also use it with the port to create an artificial harbor. By the way just keep in mind that the largest, and therefore coolest, industrial buildings are 4x4. But I've only seen, I think, 3 types. One dirty one, one manufacturing one, and one high-tech one.

I hate highways. Fortunately I've found several sites that insist they are entirely unnecessary.

That's a pity :( Highways look really cool, even though they are an absolute pain to place (or specifically the on ramps are) I was a bit surprised though by the little amount of traffic on them, even next to dense areas. I'm curious to see if will get a bit better when my city reaches 'full' size.

I put some of my thoughts here.

Subways are useful once you've run out of tiles. The tracks are expensive to build, and every five sections of track cost $1 a month in upkeep. But because you're replacing rails, that's five tiles you free up aboveground, which can easily make you more than $1 a month in revenue.

You can build subways earlier, but it will slow the growth of your city because the tracks are so expensive. Although you get access to subways very early, they're actually a late-game feature. But the early invention of subways is useful for giving you access to better train stations.

You can actually build a functional city on roads and buses alone. You need to plan your zones like a sandwich. Residential zones need access to both industrial and commercial zones, so residential zones go in the middle layer to minimise travel distances to the other zones. This also divides your traffic in two directions instead of forcing every sim along the same route. You can turn your main streets into boulevards to support the load of traffic. You can build T-junctions to reduce congestion. You can remove sections of road to divert the flow of traffic.

As for your follow-up questions, I let the city grow organically and then go back to improve the older parts of the city. This often means demolishing sections at a time to put up new zones and infrastructure. Once you hit about 80,000 population, the money just rolls in and you can afford to revitalise downtown.

Ramps Suck. Seriously. I don't mind Highways. But I'm going to try to do without them because Ramps SUCK.

They are genuinely bugged on my end. Sometimes the highlight (red square for impossible placement or green square for allowed placement) for onramps will simply disappear and then I need to select something else first like roads or demolish to reset it. But even that only works most of the time. Sometimes I simply can't place a ramp in valid position. But I usually resolve that by just moving the ramp one position further along the highway.

Tunnels and bridges are similarly finicky. It's times like that when I start to miss sc4 a bit :P

I really like this game. But the tools can get frustrating. And led to some venting. LOL.
Plus side: got some farms going and got hit by Locusts. Very Funny.

If I understand your question correctly: I haven't tested it but I don't think traffic can migrate to another track unless it's connected to that track. So if you want the trains picking up people from that spot to use both tracks then you either have to build two train stations, or connect them like this: (orange and red-orange tiles are the tracks)