I waited for a freight train to pass recently and the cars were the type that held two large containers stacked piggyback. It didn't seem there were any external restraints that held the top container to the bottom one...are they held there just by gravity?

The containers are attached to each other via pins, the two containers, however, are held in place in the railcar by gravity only.

Thanks for the info. These type cars would be hard to ride on for a tramp, huh?

Not for the most part, no.......it really depends on a few issues as to whether a tramp could ride on one or not....