Choooo-choooo. Not since Casey Jones has there been a train wreck as spectacular as the one in which the district attorney of Fulton County ran off the rails in her effort to prosecute President Trump for election interference. With each passing challenge to the way the D.A, Fani Willis, was conducting the case, she vowed to press on — saying, at one point, “I do think there are efforts to slow down this train, but the train is coming.”

Today a panel of a state appeals court in Georgia hit the brakes. It removed from the case not only Ms. Willis but her entire office. The logic for that, our A.R. Hoffman explains, follows from the fact that the authority to prosecute is vested in the elected district attorney, not in her legal staff. It’s not too soon to suggest that this represents a huge step forward for President-elect Trump and his co-defendants. It is too soon, though, to suggest the case is over.

Ms. Willis was challenged in the district court trying the case. It found that the romance she was secretly carrying on with the special prosecutor she’d hired to handle the matter was improper and that either Ms. Willis or her erstwhile boyfriend, Nathan Wade, would have to go. Mr. Wade stepped down, but Ms. Willis carried on, winning reelection last month. Just days ago, she wrote to Congressman Jim Jordan that the case “will not be dismissed.”

As late as Wednesday, Ms. Willis’s office wrote in a filing that “‘president-elect immunity’ obviously does not exist.” She also called Trump’s efforts to dismiss the case “procedurally and legally inadequate.” So it couldn’t be more dramatic that the next day, Ms. Willis herself has been dismissed from the matter and the whole vast case is grinding to a halt like the cars of a train piling up behind the engine that had derailed.  

The appeals court reckons that Mr. Wade’s departure did “nothing to address the appearance of impropriety that existed at times when DA Willis was exercising her broad pretrial discretion about who to prosecute and what charges to bring.” The panel that made the decision, though, was divided two to one and Ms. Willis could well appeal. The indictments, moreover, were left intact, and Georgia’s supreme court could yet restore Ms. Willis to the case.

We hope not — although, as Mr. Hoffman’s news story makes clear, the dissent is not without serious points. It is the job of a trial court, after all, to adduce and decide the facts. The appeals court did not — and was not in a position to — make its own factual determinations. Georgia’s Supreme Court could reckon that the trial court’s ruling keeping Ms. Willis on the case was owed more deference than it got.  

By our lights, in any event, it’s hard to see how Ms. Willis emerges from this matter in a position to prosecute it fairly. The appeals court reasoned that her relationship with Mr. Wade cast a pall of impropriety over her prosecutorial “discretion,” meaning her judgment on whom and what to charge. Justice Robert Jackson, FDR’s attorney general, reasoned long ago that a prosecutor’s “discretion is tremendous.” That cuts to the constitutional quick.

The stakes in Georgia reach beyond Ms. Willis. These columns as recently as Tuesday warned of the threat posed by state prosecutions of a sitting president. Ms. Willis was heedless of those concerns. Never mind precedent back going as far as McCulloch v. Maryland in 1819 — and cited by Trump — that holds that state antagonism toward federal prerogative is a danger. In the folk song,* after all, Casey Jones collides with an oncoming locomotive. 

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* Here’s the verse in the version sung by Pete Seeger: 

Casey got to that certain place.
Old Number 9 Stared him straight in the face.
He said to the firemen, ‘Boys you better jump.
‘Cause there’s two locomotives
‘And they’re bound to bump.’