Question about trade -- why do different cities get different income from trade?

[The Fjonis]

I've noticed in some of my games that cities that are apparently quite similar can have very different trade incomes. For example, one coastal city might have 5 gold from each trade route (which are foreign), while another city might have only 1 gold from each (which usually then are trade routes with other cities in my civilization). I am aware that some improvements etc can give increased trade income, but I don't think this can account for it all..

So, what exactly determines

a) the number of trade routes in a city?
b) the income from each trade route?

Thank you!

 

[Agarwaen]

The number are affected by some tecnologies, a civic and some wonders
Liberalism adds one in all cities and great lighthouse adds 2 in all coastal cities for exemple
currency adds one in all cities too
and in the exp the Castle adds one untill cannons or something like that

 

[Mewtarthio]

The income per trade route is determined primarily by the distance between the cities, with a significant bonus for foreign trade routes. Population is also taken into account (on both ends). Note that trade routes are constantly recalculated to maximize income.

 

[CivDude86]

You can only have 1 trade to a foreign city and hardors increase trade route yield by 50% so thats why they get all the good ones.