I agree that walking time, speed and frequency are the three variables you can use to influence the maximum travel distance. However the difference between using ‘frequency’ or ‘frequency/2’ as waiting time is pretty significant to judge the viability of a given line (especially trains) as you need to run long trains (consequently at lower frequencies) to get a good return on investment.
I appears that for longer lines the frequency calculation stays the same, i.e. total time for one round trip divided by the number of vehicles. I think the game (thankfully) assumes that our vehicles run at perfect separation. 🙂 However, there does appear to be some timing mechanism to determine the actual roundtrip time, once the line is actually running.
With regards to cargo, it seems that this is purely time-based (at least for the primary industries). In the example I mentioned earlier, the oil well’s one production / year spawned exactly on January 1st. 🙂