Thanks for the feedback! A small percentage of people have had the similar feedback that they've not realised they can go inside the train to find more metal nuts (or didn't realise immediately).
That is one of the graphics that the game jam deadline left little time to work on more, but I am planning to return with some tweaks in spring after the Jam review has taken place! I hope that you'll consider giving it another go, there's so much more to see.