An argument typically has one of three outcomes. Rarely, one person convinces the other on a certain point. Somewhat more hopeful, both people retain different views, but both opinions are enriched or gain nuance through exchanging ideas.
"(A) difference of opinion" depicts a more common outcome, two people could talk endlessly and never agree. The artist suggests this is like two parallel train tracks in a particular railroad section that fall into disrepair because they do not connect at any useful junction. Therefore, in discussion, we need to seek out points of agreement eagerly, not diligently maintain our opinions only.
