#3 Mandie and the Ghost Bandits by Lois Gladys Leppard, 1984.
Mandie and her family are going to proceed with the plan to create a hospital for the Cherokees with the gold that they found in the cave. To do that, they need to take the gold to a bank in Asheville. They decide to take the gold secretly on a train. Uncle Ned stays in the baggage car to watch the gold as they set out on their journey.
When the kids decide to leave their car to go visit him, strange things begin to happen. They see riders dressed as ghosts out the window. Then, the car with Uncle Ned and the gold inside is disconnected from the rest of the train, crashing into a ravine. The train comes to a stop, and the kids jump off to see if they can help Uncle Ned.
Before they can find him, the train starts up again, leaving them behind, and they are taken prisoner by the mysterious ghost bandits. They are after the gold in the baggage car, but how did they know it was there when Mandie’s family tried so hard to keep it a secret? Mandie and her friends must escape and find the others before it is too late!
In some respects, I would call this more of an adventure story than a mystery, although there is still the question of how the bandits knew where the gold was in order to steal it.