There is also extra gold for each railroad tile the trader pass by, for passing by water, for each trade post it goes through, and for (every?) tunnel.
Because the OP just made his first post and sounds like he's relatively new to civ6, I will clarify this statement.
When you make a trade route, there is going to be a "base value" (essentially what comes from districts) and then other bonuses (from cards, great people effects, wonders, etc.)
For international routes only, there is what's called a trade route efficiency mechanic that can increase the
base gold value of the route. The way it works is, the different tiles and features along the trade all have a number of efficiency points associated with them; the total points for the route is divided by the length of the route itself. That ratio determines the bonus, which ranges from 0 to +100% of the base gold value.
Land tiles by default all have a score of 1. Since each tile adds 1 length to the trade route, you will never beat the minimum ratio of 1:1.
Water tiles by default have a score of 2. This means that having more water tiles in your route increases the ratio! The +100% bonus point comes at an efficiency_score:route_length ratio of ~ 1.6:1.
Railroads also have a score of 2. So upgrading your roads to railroads in the industrial era with Military engineers can boost both your unit movement and your trade routes - but only the
gold on
international routes.
Going through a mountain tunnel adds +15 efficiency score. This means that if your trade route is ~9 tiles or so, traversing a single mountain tunnel will give a land route the full bonus.
Note: yes, this applies to the Incan unique
Qhapac Nan mountain tunnel, so they can get some nice bonuses very early.
