how much of that 35 minutes is travel time and how much waiting time? did you also check for the real trip time or did you just calculate it from the frequency? because it seems that when you start a line the frequency is often stated a little higher than it actually is.
the results from my own testing are a little irregular. i seem to be able to sometimes create rail lines, which despite having a trip time of around or over 20 minutes (derived from the frequency) are used to transport cargo, but only if the frequency is ridiculously low. so frequency might to have an influence, although its difficult to say how it works really, especially since it seems that with a trip time of well under 20 minutes frequency seems to have no influence at all.