Personally, I do like mail and passengers.
1. As You transport more, the cities on both ends of the line grow, so, You just put more trains at no infrastructure cost and Your business grows.
2. Even, if You transport coal or oil, You usually at the start transport it only 1 way, passengers and mail run bidirectionally, so it is recommended to peer-up two cities of a similar size.
3. With cargo it is difficult to have efficient train utilization. I usually tier this out: primary resources, like oil/farms output/wood: slow diesel. Then for secondary level cargo one can use better transport means: electric/monorail/maglev. However, pay attention to properly feed the converting industry to properly saturate the transport of goods and similar, secondary cargo.
Also, please take a look at the cargo rates, where some cargo types pay more when they are transported fast, some can take time and still pay off.
The ideal setup for cities is to find a set of 6:
3 @ one corner - 3 @ other corner, connected with long diagonal path. So, You can triple-utilize the main line, and pair cities on both ends of the map. Sometimes cities are big enough to just be two pairs. Remember that collision-free crossings is a must then.
Ideal setup for coal:
3-4 coal mines - diagonal route - power station - 3-4 coal nearby coal mines - diagonal route - power station - (and again).
But, then using "Full load" option will kill the rating. You need to use schedules i.e. 5-6 days per coal mine station.
That should give "full" trains going both ways. So, even if they run at 70% utilization, this still brings more money, than one way 100%.
Having said this, as during scored plays, time is the largest constraint: build infrastructure wisely, with a though to reuse it or scale for future growth.