Duplo Locomotive: What is this inner part used for? Background

I was forced to completely disassemble a duplo locomotive. My son played with it in a sand box and, guess what, it was not working anymore. I used a small brush to clean every single part (mainly axis and gears). I eventually managed to get rid of all the sand. Then I reassembled the locomotive.

Questions

I ended up with 2 small parts I could not figure out where to put them, neither what they are used for. Everything seem to work without these 2 parts - my son is happy again. These are curved parts, with a spring attached to it.
They are on left part of this picture:



So what are these 2 parts used for, and where should I put them ?
They are protective locks. If something (let’s say kids hair, or part of their clothes) gets stuck and dragged inside the wheel, these tiny blocks will lock the wheels and make sure it doesn’t keep dragging whatever is stuck. As soon as you pull the stuck part back, the springs unlock.