This was an awesome movie!!! I literally watched it two times back to back because I didn’t realize at the very beginning the man on TV running for office was actually the adult son of LeAnn and Bayou, which is why LeAnn couldn’t have the baby at the hospital, she didn’t know if it would be born with darker skin. However, the child was born with very fair skin so he passed for white all his life, and no one ever knew, to include him nor LeAnn’s husband, knew that he was Bayou’s son.
The old lady walking on the railroad tracks is Bayou’s mother now older, who knew everything and even delivered the baby. So she took the letters to City Hall to her “grandson” so that he’d finally know the truth. There was no murder to be solved, everyone knew who killed Bayou and why, they were all there. I think Bayou’s mom wanted her grandson to know the truth before she died.