Peter Hamilton and Charles Dow stressed that, in order for the primary trend to be valid, both the Industrial Average and the Rail Average (now the transportation) must confirm each other. If one average records a new high or new low, then the other must soon follow for a Dow Theory signal to be considered valid.
PS: Why the rails? There is no doubt that today's economy is very different; consequently, the makeup of the DJTA has changed to favor the airlines. However, there is still some credibility in using the DJTA to confirm movements in the DJIA. Transport stocks are much more dependent on the economic environment than the average stock and will likely foreshadow economic growth.