Example:
A train only carries food, after unloading and flipping all available cargo types that the waggons can carry are shown, you can see that in the vehicle UI details tab or the 1st person camera view UI at the bottom.

The side effect is that the power calculation of the train is based on the pre set cargo types even if the waggons are empty.
This means the readout of the engines capabilities is wrong when using mods that change the cargo weights (in vanilla everything weighs 1200 kg per cubic metre).