As someone who loves puzzle/problem solving, I found the Shunting Challenges to be extremely fun - if not a little TOO easy. What I'd love to see in a future update are more Shunting Challenges added, perhaps using some of the yard areas that are rarely used, or not used at all, such as Crovan's Gate Works Sidings, Timer Mill Sidings, and especially Wellsworth Goods Sidings. I also think further shunting challenges with increased difficulty would be a great addition. Perhaps "Knapford Yard Shunting Challenge 2" could be arranged with the numbered trucks in different places, more spaced out from each other, with a larger amount of non-numbered trucks in the way, giving a more "Medium" difficulty, and then "Knapford Yard Shunting Challenge 3 could be even more complex, like a "Hard" difficulty. Also, making the "Welcome to the Shunting Challenge" intro skip-able would be a godsend!
I'd definitely like more. I found them quite frustrating at first, but once I got my head round them (such as realising you're not meant to put ALL of the trucks into one long train...) I found them to be rather relaxing due to how methodical they are.