G'day Dan - thanks for your response. In my case I've got 2 trains leaving from staging A and both going thru the same town B but at different times.
My Build action shows that each of them is being called to pick up the same 2 cars in town B on their journey. I've checked my settings and car options for each train and all seems OK.
In town B there are a variety of other car options to pick up, so not sure why these 2 trains want to pick up the same 2 cars!?
?