Hi Doug - so probably what happened is your rail revolve result was a solid that had an end cap on the bottom.

Join only tries to join together open unattached edges between things, so your revolve would need to have its end cap deleted (or uncheck the "Cap ends" option so it's not created in the first place) in order for it to be set up for join.

Basically Join does not try to intersect objects or remove material, unlike the boolean commands.

- Michael