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AFAIK this is the most definite answer to "perfect distance" you can get: citymania.org/tools/profit

As for passengers, there are several factors that make them so good.
1. It's essential two-way cargo. While you can do two-way with other cargos it requieres a more complex setup, and incurs more transfering delays, so payment is less.
2. Passenger wagons are usually quite capacious, so even though payment rate for 1 unit seems low, it gets better when you calculate for 1 wagon. And towns (well, if not too small ofc) generate a lot of passengers, much more than any industry.
3. Passengers weight less, so trains accelerate faster, travel time is less and payment is better.

Though, while passengers bring a lot of money, I'd say they aren't the most profitable cargo. Usually you can do better with, say, goods in a long run. But pax/mail is perfect for starting route as it pays more for the same amount of effort.

That graph was cool! I am going to use it. Doesn't have all industries but it show where something is profitable. I have started making my own now as well, some troubleshooting and it should be finished. The reason I got into this is that I tried to start a map with fruit once and I got 45k out of it. With oil I can get 95k per trip. I just got to curious.

The reason that passengers are so highly rewarding is because you deliver them two-way and later in the game when the cities are big the production is ridiculous.
Anyaway, maybe this is the tool which are you looking for. You can change freight multiplier, train length and locomotive and it provides you some income comparsion with optimal travel lenght

citymania.org/tools/profit

Sadly there are not all types of cargo.. but I guess it is still worth lookin