In one of the earlier replies glass sinad was also mentioned as being carried by these cars. What is the difference between frac sand and glass sand?
Based on what I've read about frac sand and a (very) rudimentary knowledge of glass making, I'd opine that it comes down to a couple of things.
One, the frac sand has a certain shape and size that makes it conducive to holding open the fissures created by the high-pressure injection that is hydraulic fracturing. Hardness may also be a factor.
With glass sand, it's the chemical/mineral composition that's important in order to gain the expected results. The size of the grains and and any other physical characteristics aren't really considerations.
That's not to say there couldn't be crossover.