For example, we have a basic open wagon which can be loaded with 8 coal or 8 iron and etc. Fun thing is when at a station where both coal and iron await it loads both 8 coal and 8 iron. It is nice having twice or even thrice more loaded cargo in a single wagon, but it not fair! 🙂 .
Also i’m not sure, but total mass of a train can be calculated incorrectly in this case: additional weight of overloaded cargo can be omitted in calculation.
I could understand that we can load x items of coal and y items of iron, where x+y = 8 , but even this is not done at practice. One open wagon – one type of cargo.