Like AC said but a little more. To recap: Cold solder joint can also be from not heating up both parts, I.E. wire got hot enough, but rail did not. Since the rail did not get hot enough, solder will not flow proper.
Can also be because one of the connections is dirty; hence not good heat transfer. You can always tell - A good solder joint is bright and shiny with solder flow on both connections..bad is a dull gray with bubbles or space between ( in this case ) the rail and wire; you may also see remnants of flux between the joints too.
And always remember to grab one of the connections ( if possible ) and give it a yank to test the mechanical connection.
L8r