This is a really fun game as is, and with some more visual/auditory polish it would be a homerun! Love the core mechanic its very engaging and super fun to build up your train station and which the shapes zip around. Some of the little details are excellent and really add a lot to the experience, such as the pentagon rotation when a train goes through the hub. The sfx work well along the same lines, but some subtle atmospheric background music is sorely needed.
Having challenges was an awesome touch and kept me engaged and grinding longer than I would have otherwise, and the ending message was a nice little bit of thoughtfulness and use of the theme. The UI could use a bit of work (understandable for a jam) but the overall minimalist aesthetic works really well I feel. A couple of minor criticisms: you should be able to delete nodes for a cost, clicking on a train line should bring up the upgrades for that line immediately (without having to go through the list), and the color-coding of the upgrade buttons didn't match the train color and was often too light and as a result barely readable.
That being said this was a very enjoyable playthrough and I think has a lot of potential as an overall game. Really fun gameplay loop with the upgrades and a very pleasant minimalist aesthetic. Great entry!!