WMG: Mario & Luigi: Superstar Saga

Click here for Wild Mass Guessing for the series as a whole.
The Mine Cart Owner is a ghost.
When you beat the Mysterious Mine Carts, the old guy is seen fading into the image, then remarking "this spot marks the last stop on the long, strange railroad ride of my life". It seems likely that he refers to that spot (the end of the minecart ride) being where he left the mortal coil.
The Mushroom Kingdom's economy is in the crapper.
This explains the ridiculous exchange rates and the rather big inmigrant community in Beanbean Kingdom.
Fawful and Popple are relatives of some kind.
Think about it... Their names rhyme, they both cause mischief and mayhem wherever they go, and they both have something wrong with their head (Fawful is an Talkative Loon, while Popple is a Captain Oblivious to ridiculous extremes). It would make sense. It also helps that Fawful used Popple's boss song for his last fight in Superstar Saga.