Why is it that the majority of open hopper cars on the market all have coal loads ?.
I sat and watched a freight train go by at a local crossing the other day, and saw a lot of open hoppers carrying white rock and so forth, but didn’t see a single one carrying coal or anything that even resembled coal…
Tracklayer
Depends on where you are, but the vast majority of hopper cars were used to handle coal. The majority of hopper cars made by model manufacturers are cars that were designed specifically for coal. They are sized so that if you fill them full of coal you will have reached the capacity of the car. In other words if you take a car like the Athearn twin and fill it will coal you will have 50-55 tons of coal in it, which is what its capacity is. If you take an H21 hopper (the Bowser PRR quad) and fill it full of coal you will have 70 tons of coal in it and its a 70 ton car. Normally rock is denser and heavier so you have to have a higher capacity car or not fill it full. If you take an H21 and fill it with gravel to the same level you do with coal you will overload the car.
Dave H.
dehusman makes a good point. The majority of hoppers out there were designed to haul coal, but there are alot of special use hoppers to carry other commodities as well. For instance, alot of older coal hoppers were converted to carry wood chips by adding taller side panels, because wood didn’t weigh as much as coal you could haul more volume load on the same car. There are also different cars for hauling gravel and ballast. Due to the heavier weight, they can’t be used in traditional coal hopper cars.
All hoppers may be somewhat similar in appearance, but most are purpose built (rotary dump coal hoppers, bethgon coalporters, greenville woodchip hoppers, ballast cars, ore hoppers, etc). The reason most model manufacturers flood the market with coal hoppers is because that’s one of the most popular type of car on today’s model railroads. Everybody’s gotta have a coal mine on their layout somewhere! 
It’s the lifeblood of America!
That’s why I’m planning both a coal mine AND a rock quarry. I love open top unit trains of coal, but we also have quite a few gravel quarries around this area, so ballast trains are also seen from time to time, and I want to include both on my layout.
Thanks for straightening me out on this guys. I’ve been wondering about it for a long time, and when I saw the freight train the other day with the rock loads it brought the issue back up.
Tracklayer
To add to dehusman’s point. Todays 100 ton hoppers might look the same to a casual observer but a check of the cubic foot stencil will reveal that the typical CSX or NS hopper is slightly smaller than the typical BNSF hopper. Why? Appalachian coal is denser than Powder River coal.