Yes, I did derive the travel time from the frequency – it isn’t always completely accurate, but I think it’s the same data the game uses to decide whether to use a cargo line or not. 🙂
The most exaggerated example I found was a line with a travel time of 19 mins: one vehicle, no cargo. Two vehicles (freq. 19 mins). Of course, if the travel time is much shorter than 20 minutes, you will always stay under the 35 min and you can basically use any frequency you want.
I have derived all my data from trucks as they are easy to set up next to the industries (to minimise walking time). Do your trains deliver straight from industry to industry? If there are any feeder networks in between you will of course have to include their travel time as well!