During the train-ride north, Bob Younger is playing the "Jew's harp", (also known as "jaw harp", "mouth harp", "Ozark harp", "trump", or "juice harp". He is playing a song called "The Wayward Boy", of which Jim Younger is heard singing a few verses.
The gentleman bothering Cole Younger (David Carradine) outside the bank just before the shootout is speaking Swedish. He asks (in Swedish) if he can buy Cole's horse, and is refused. After being refused the Swede replies, "A shame. It's a beautiful horse".