The Dow Jones Industrial Average ( ^DJI 0.00% ) is just one of several Dow Jones market trackers. The secular index will never invite any tickers from the railroading, airline, or electrical utilities industries -- because there are separate Dow indexes for these sectors.
If you want to track these industries, you'll have to look for the Dow Jones Transportation Average (DJINDICES: DJT) or Dow Jones Utility Average (DJINDICES: ^DJU) instead. There's even a composite index that brings all 65 tickers together.
In the video below, Fool analyst Anders Bylund explains why you'll never see Duke Energy ( DUK 1.33% ) and its $48 billion market cap on the DJIA, nor the even larger Union Pacific ( UNP 2.34% ) railroad.