As far as which goods each city accepts, I’m pretty certain that it’s just first come, first serve.
If the entire chain is within the 20 minute window with each individual line, it should come to life within a year or two and then slowly pick up and upgrade as needed (and certainly start shipping the measly 25 raw products to the nearest station)
The biggest traps are setting vehicles to “wait for full load” – which increases the travel time since it’s just sitting there doing nothing, and not linking up enough cities to a single factory. At the start of the game each city rarely accepts more than 30-40 goods each. That’s 4-5 cities to get to 200 production for the entire chain.