(figuratively, idiomatic) A talk about one's pet topic (or the topic itself), especially when only tangentially relevant to an ongoing discussion.

Example:   The madman obtained a soapbox which he stood on at the corner of Broadway and Wall street, to shout out his prophesy of the end of the world.
2013, Robert F. Ely, Candidate for President (ISBN 148175582X), page 25
  Bernhard's last appearance on a late-night talk show was a handy soapbox to expound on his political message of patriotism, nationalism, and populism.
  He's been on his soapbox all day about the new football coach.
A soapbox car.