From #openttdcoop wiki
In several GRFs it happens that several locs are limited to a certain kind of cargo, and that the fastest trains are pax-only. So if we wanted to have those trains coexist on the same network, then we should make sure the fast trains can follow their way without experiencing much hindrance from other trains.
To obtain that, a mainline would be at least 2 wide in one direction, and lane to the left would be reserved for faster traffic. Since speed can not be measured absolutely, a system like this would only work when there is a difference in speed. Trains will therefore use one track while unhindred, and only when encountering a slower train will they decide to switch lanes.
As a nice testing environment, a proposal would be: Connect towns, ICE/S-Bahn style. Build feeders to central hubs and transport pax across map Connect a food plant with the farms on the map, and distribute the food to above-snowline villages to help them grow. Trains share the same network - the slower food trains however prefer the right tracks and the faster pax trains prefer the left ones via the speedswitchers