Mitchum is both the tragic hero and chief villain of the Tunnel Disaster. A man who got his job through personal connections and shady dealing, Dave Mitchum has never made an important decision in his life:
Dave Mitchum had spent his life slipping around the necessity of ever making a decision; he had done it by waiting to be told and never being certain of anything. All that he now allowed into his brain was a long, indignant whine against injustice. (18.104.22.168)
What's scary about the looters' regime is that so many people in positions of power are just like Mitchum: none of them are willing to make decisions or take responsibility for things. And in a society where no one is willing to decide anything, incidents like the Tunnel Disaster happen while people essentially stand around shrugging.
Mitchum not only represents the type of unqualified people getting management positions in this society; he also gives us a compelling portrait of how anxiety and fear can lead people to make huge mistakes. Terrified of reprisals from Washington, Mitchum allows the train to proceed through the tunnel with a dangerous engine, the final result of a sort of domino effect of fear.
Dave Mitchum is trying to save his own skin above all else in a society that is always on the hunt for scapegoats. He doesn't want to make any decisions at all, and this attitude (shared by the majority in power) puts him in a no-win situation where he has to do something he doesn't want to.
What's ironic, and sad, is that Mitchum doesn't even realize he has a choice in the matter: he could refuse to run the train. In the end he is too scared of Washington and too incapable of thinking independently to make a good decision, so he runs away and tries to pawn the blame off on someone else.