Hey,
I think it is randomised to a certain point. Kind of "2-3 early era goods, 2-3 mid-era goods and 2-3 late era goods".

For example in era 1, you have :

cows and boards for level 2 and 3, in one or the other order.
iron ore level 4 usually. iron and cotton for level 5 and 6, and thread and leather for level 7 and 8. But these combinations of 2 can pop up in one way or the other.

For era 2, you will most certainly have quartz at level 15 ou 16, same for copper ore. Whereas meat and paper will appear quite early.

I think that it might depend on the ranking of your city a bit, but I'm not sure and I've not paid close attention to it across multiple servers to collect enough data.
A nice way to actually see which goods are possibly available at which level, the Europe server, if you are the mayor !

Sometimes Rail Nation like to surprise us, and when we think its a certain good it is something else.

I hope that you managed to get your tracks down, and saved for all your wagons. If you haven't completed your End Game, good luck!

Emziie.