Have you ever been walking along a railroad right-of-way and found some glass balls? Through a combination of some newsgroup reading and firsthand experience of one of our members who's gone on to work for BNSF, we can tell you this: These balls are the raw materials used in the making of fiberglass. They are sometimes carried in open gondolas, so they tend to bounce out. When the freight car reaches the plant, the balls are unloaded then melted.
John Klemin offers the following opinion: "It is my understanding the RR's used the glass balls as casters under heavy freight that needed to be moved in the late 1800s. They would put 2 rows of 2, 1 "inch laths close together, fill the gaps with the marbles and push the freight up unto the marbles and then where ever they wanted them to go."
If anyone has additional or different info than this please let us know!