This is an interesting and clever way to expand a traditional idle game into a fun and charming little game!
First, I wanted to talk about the art and your use of limited shapes. I actually quite enjoyed the minimalistic and colorful art style, and wanted to shout out your menus and UI design, which was thought out very clearly and was very logical.
However, I saw a lot of rectangles within the menus + a big oval in the BG. I unfortunately don't think this would qualify for the shape limitation.
As for the gameplay, I enjoyed it surprisingly a lot! Idle games usually aren't my niche or area of expertise, but the intuitivity and unique feel of this game definitely hooked me. The missions were also quite a nice addition and kept me engaged. I also like the animation of the trains [dots] moving between stations, it made the game feel alive!
If you're going to keep working on the game, here's what I'd recommend working on:
1) Make sure you are using only circles and pentagons! I know from experience it's quite easy to overlook and sometimes slips under the radar.
2) Consider adding more sound effects and music! It's a really easy way and great use of time to spice up your game without having to take much time or effort.