Hi Tom,
You have a bit of a quandary here... the finish of the wood has to be believable in the context of the underframe colouring. As the wagon is at the moment the solebars have a clean appearance and could be not long out of shops... in which case the decking is appropriate for some hardwoods (eg. Utile). If the decking is newish pine then the wood ought to be of a more yellow colour. Weathering / distressing / ageing the blue paint shall move the decking colour towards grey / silver.
As Mickoo has observed, some loads are going to leave their mark quicker than other loads... for example:- oil spillage is going to stain the timber very quickly whereas timber loads are going to darken the wood more gradually.
regards, Graham