Memories are a funny thing. They can be fun sad and all points in between. Seeing a blue sky on a nice warm day can trigger a memory like sitting on a rock in the South Fork of the Yuba River. Sloshing around a gold pan trying to pick out rocks and trying to wash the gravel out of it till I get to the bottom of the pan. There you see black sand and if you have gotten the right dirt some flakes of gold or even a nugget.
Hearing a train can make me think of growing up close to tracks that you can here it clicking down the tracks and blowing a whistle because you have the window open to try and catch that cool evening breeze.