While we wait for further details, some ideas;
1. A double track mainline like your HO layout will allow two trains continuous running in opposite directions.
2. A yard along the way will allow you to: 1) take a train out of action, 2) perform route switching to revise the consist for variety (and fun), 3) substitute a different train because we all own more than we can run simultaneously.
3. I hear you say you mostly want to watch them run, but please include some spurs. These can be mail/express/commissary/coach tracks for passenger, industry spurs, or even company sidings for coal, sand, and oil. If you have not played the switching game, you don't know what you are missing. In addition to the fun of switching, these moves change up the consists on the main line for more variety and fun.
4. Personally, I am deeply engaged in passenger switching, in which consists are switched to match B&O prototype (but not exact car types). Great fun. Highly recommended!
