A really cool idea! The layout of the UI is nice, as is the concept. I think other comments have touched on the cons of the game, but I'll add that it's an accessibility issue for me — the awkwardness of track moving and placing is rough on my tendonitis, so I couldn't give it a very long try. I also noticed the hitboxes on tiles for detecting the mouse seems inconsistent? And, frequently, I got combos of starting tiles that were impossible, as best I could tell, to create a new track with.
In a post-jam version, you might consider making the early tiles easily fit into the spots left behind by the initial track pieces, so the first few swaps are simple, and the player can expand the loop to earn a little more thinking time fairly easily. If it was more feasible to plan out loops and then quickly place them, it'd be much more fun (this is the only way I was able to score any points at all — definitely keep the ability to place track that doesn't connect, you can plan in areas adjacent to the current track!)
Also consider perhaps some kind of limited pause for dealing with tricky areas where a swap might need to be extremely fast. Perhaps you could allow one pause per lap, or restore the available pause every real-life minute or so? And then allow placing a single piece during that. Combine that with easy swapping of pieces, and it might take care of a lot of the tougher bits that come from random generation.
Regarding the starting tile issue, I don't know a way around it other than either making tiles the same size as the grid and all the same size, or writing some validation code that can check if a new track loop is possible, regenerating all available pieces (or just one piece at a time for those gained during gameplay) if a different loop isn't possible.