David Simon talks about it in the commentary track.
They pretty much forced him to do that, they were worried that people wouldn't remember that particular character.
Similar to this...As far as I know, David Simon has yet to talk about what his train track metaphor is. Throughout the entire series and post finale he would point out that no one writing about the show was able to decipher what he was trying to say when Bunk and McNulty have their talks at the rail yards.
Interview from 2006:
Q: We heard about your frustration over the critics not noticing the train symbolism. What else don't we get that bothers you?
A: Frustration is overstating it, perhaps. But I am often surprised that sometimes the most fundamental thematic symbolism is ignored in television, where in a feature film it would be acknowledged and discussed. So much of television has for so long been filmed in a rapid-fire, utilitarian fashion that when filmmakers attempt to use the medium in any kind of visual or allegorical way, it rolls right past many people. This is true of some other well-made dramas on HBO as well.
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Interview POST Series Finale:
Q:I'll start with the obvious one. The show's ending, this is your last chance to do this: What the hell do the train tracks mean? (NOTE: Simon has in the past expressed surprise that no critic has ever correctly interpreted the symbolism of why McNulty and Bunk's drunken bull sessions usually take place beside train tracks.)
David Simon: No shot. You're not getting it out of me.
Q: Oh, come on!
David Simon: To talk about symbolism, if people get it, they get it. if they don't, telling it to them ruins it. You know that.
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This random internet posting is what I assume it means:
The key signifier in the show was Bunk and McNulty getting drunk by the train tracks. The train train tracks symbolize the unalterable steel shod inevitability of fate and the institutions that cannot change direction. And perhaps the key scene in the entire series was McNulty urinating on the tracks while the train approached from behind him, unaware in his eagerness to stick it to the powers that be that the only choices he has are to get out of the way or be crushed.
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