There are two types of transmission lines as they relate to loudspeakers. One is acoustic and one is electronic.
WIKI – Acoustic Transmission Line
WIKI – Transmission Line Loudspeaker

The electronic version allows for time correction between woofers and tweeters in a passive speaker design. Using inductors and capacitors, you can delay which ever loudspeaker arrives early (typically the tweeter) and phase align it with the woofer.

I have yet to find a good internet link to explain the electronic version. More soon…