Actually the AI traffic in railworks should be called ADHD traffic, it's all dependent on timing and consist priorities, and doesn't pay any attention to signals at all. The scenario editor is ridiculously clumsy, complicated, limited, and buggy, in my opinion the MSTS route editor is more stable and easier to use than the railworks scenario editor. The kid who designed it had a specific type of activity in mind, and built the logic and tools around that type of activity. Even that didn't work well at the beginning, so with the very first patch 3 years ago they redesigned it so the AI trains locked every single switch on their entire path at start time, even if they weren't due to start running for 4 hours after the scenario starts. Consequently the entire scenario has to be set up with priorities and timing letter perfect, no deviation from the schedule is allowed, and even then you get player collisions with invisible AI trains and AI trains colliding with each other unless you have a multitrack mainline passenger route with a limited number of AI trains all running on their own tracks with no conflicts - in which case you don't actually need signals in the first place, they're just for decoration.
So the short answer is no, you can "design your own scenarios for current routes and whether you can give commands to AI trains" but nowhere near "just like all the "Trainz" have let you do?", railworks isn't even in the same class as Trainz when it comes to ease of use, versatility, and stability of AI traffic setup. In fact the 9 year old MSTS activity editor is actually more stable, versatile, and intuitive than the RW scenario tools. MSTS AI traffic, while nowhere near as good as Trainz, is actually better than railworks.
As for RW having more freeware diesels than Trainz, I'm more into first generation stuff like Alco RS3 and EMD GP9 diesels, not interested at all in SD70, FP40, or Dash 9 modern stuff. Possibly RW has more modern US diesels than Trainz, or more European stuff, I never really looked.