Several people were asking for Cloud Saves so I had to read up on it and it turned out that Steam provides a simpler version of cloud saving called Steam Auto-Cloud where the developer doesn't have to figure out the API and write arcane code. I was able to test between a Mac and Windows machine, but I need feedback from people who have a Steam Deck on whether their save file is syncing between their PC and Steam Deck.
Fyi, the save file path on each of the machines are:
Windows
%USERPROFILE%/AppData/LocalLow/Gamer To Game Developer/Choo Choo Survivor
Mac
~/Library/Application Support/Gamer To Game Developer/Choo Choo Survivor
Steam OS (I think this is Steam Deck)
~/.config/unity3d/Gamer To Game Developer/Choo Choo Survivor
Let me know if cloud saves are working for you, especially I'd like to hear from anyone who plays between their PC and a Steam Deck. Thanks.