Activities and timetables populate the tracks with trains and stock which your train must work with. Well-designed activities challenge you to operate your train effectively and evaluate your performance. Activities are also useful in providing tutorials to help you get the most out of Open Rails. See Australia's Zig-Zag Railway for some very effective steam-based tutorials.
A great place to start would be the free Siskiyou Route from the USA. The creator, Dale Rickert, has assembled a package of instructions, examples and even videos to show how to create both simple and demanding activities.
The quality of these diagrams reflects the effort that Dale has put into creating this resource.