I tried to get into all 3 cities, but I didn't quite fill up all of them. Norwich Hills I think is the city that has the hills. It has oil, coal & ore. The oil is right at the city entrance so I ended up building on top of it before I noticed it was there. In that city, 75% of the land is on a plateau. So in that city I had residential near the entrance, and one corner of the upper area. In the rest (50% of the total map) I had a coal mine/iron mine and a bunch of factories. The iron/coal mine require a trade depot which will store & export any resources you produce. I didn't really get too in depth with the mechanics of trade or refining the raw materials, as I wanted to move onto my next city.
Clearwater I had a high population city/education. I got to ~90,000 near the end and there was maybe 20% of the map that had little/no development. This town didn't have many jobs, so a large portion of my people took the railroad to Norwich Hills. The roads around the railroad lead to a huge traffic jam, due to shear volume. Fortunately two of the roads nearby were avenues so I upgraded them to streetcar avenues. I made an 'H' pattern out of my streetcar rails, and that alleviated a lot of the problem. I also put a fair amount of effort into education. I had a fully upgraded elementary school, a fully upgraded high school, and a community college. Towards the end of the session I started to get mid-tech industries forming in that city. I even got a notice that I could build a processor factory (a factory able to turn metal & plastic into electronics). With about 5 minutes left, I had ~$130k saved up, so I cut all of my taxes and there was a significant boom. Every other block had a new building in construction. I was just beginning to get some high wealth buildings, though didn't get past medium density.
The third city was a gambling city. I took out 3 loans to build up fast. I basically plopped a casino across the street from the railroad station with only a basic city infrastructure around it. It very quickly started turning a profit as tourists from my other cities would pour in through the railroad. I built a second casino further away and it too was able to become profitable after a short time. This city overall got barely any development as I was focusing on the gambling & just didn't have time.
For railroads stations, they have to be plopped next to a railroad, and they need a fair amount of room. If you already have your city in place it can make placing a railroad station difficult, and making the road system around it look nice even harder. You are able to lay railroads yourself so you can run them where you need them and attempt to make them work with the rest of your city.
As others have said, being able to exchange services is very nice. Having a coal/oil factory in another city and being able to buy electricity at least to start was a big help. I don't know how much electricity costs to buy and how profitable it is (especially if prices on the market start to fluctuate) to focus on providing services to nearby cities. I imagine being able to work together with the upper tier emergency services (hospital, large fire staion etc) will be very helpful.