Database relationship refresher
Journey.CustomerIDis a foreign key to Customer.CustomerID. Journey.TrainNo is a foreign key to TrainRide.TrainNo. 

The relationship here is many to many. A TrainRide can have many customers. A customer could ride many TrainRides. The table Journey is used to associate the two. You can see the same customer more than once in this table such as 834 or the same train more than once such as Amtrak 333.