In order to make profitable high speed lines, look how the real life works: Small local lines (ie busroutes inside a city or commutor trains to nearby small cities) that 'feed' the high speed line.
Imagine a big city with a big central station with many long range high speed lines. If you live in the outskirts of that city you would have to transport you self to the central station in order to take the high speed train. In the (standard) simutrans world, passengers are lazy, they only want to walk to a station if it is within the station catchment zone (press "v"), that means that only the passengers from the catchment zone of the big central station will use the station, the rest of the city will not be able to use the station. Therefore, local traffic is very essential, because that means that more of the city is covered by you network.
Note that this also replies "in reverse" for the passenger destination. If a passenger wants to do a jurney, but there is no local traffic in the other end, chances are that the passenger gives up.
The more cities you have on your map, the more possible destinations a given passenger may have, and thus more important that you create local traffic in your connected cities.
Hope this was of some help!