I have a freight line with 3 stops as below.
Coal --> Iron --> Steel Mill
A train hauling 6 open wagons (4 coal / 4 iron each wagon) start from the coal station, and it end up carrying 15 Coal and 11 Iron.
26 units is loaded on a 24 units capacity train?
Or each open wagon can carry 4 coal AND 4 iron at the same time?

Each category of material is a separate capacity (weird, I know.)

"I can't fit another piece of coal in here no matter how hard I try, but a few tons of steel, oil, and lumber? No problem, plenty of space, still!"