Interesting!
I did some further testing and it appears that cargo is indeed affected by frequency, but is willing to travel much further than passengers are.
Based on a few different testing lines I gather cargo is willing to take about 35 minutes (give or take 1 or 2 minutes, tested on medium difficulty, medium map size) to get to its destination, rather than the 20 minutes for passengers.
Even in 1850 this makes possible some quite long freight lines with just horse carts!
Hope this helps anyone.
Reblet