In 1864, a train's first-class carriage was discovered to be empty of passengers but liberally smeared in blood. Some hours later, the original occupant was found--dead, his body discarded near the train tracks. The police tracked his stolen top hat and watch chain through the pawn shops of London, and quickly zeroed in on a suspect: a poor German tailor. But by the time they discovered his identity, Franz Müller had already gotten on a ship to America (currently in the throes of their Civil War). The lead detective tracked him down and, though there was some political & legal trouble over the extradition (both sides in America wishing for more aid from the UK at the time, and insulted that they weren't getting it), brought him back to London for a swift trial. On the basis of his owning a hat and watch chain that were probably the banker's, Müller was convicted of murder and hanged.
This is mostly useful in revealing the types of investigative, journalistic, and legal procedures of the time. The detectives were hampered by being a fairly new profession (established only twenty-two years earlier), and still without the ability to even definitively tell animal blood from human. So instead, they mostly relied on evidence that modern courts would call circumstantial. Meanwhile, the papers went mad for this murder, to the extent that mobs waited for hours for the chance to see Müller. And in terms of the trial, the accused was not allowed to speak in his own defense, and trials were customarily very short.
The truth of what truly happened that night in 1864 may never be known--certainly Colquhoun doesn't really know. So for readers looking for a murder mystery, this might feel a little dissatisfying. But as a snap-shot of mid-Victorian English justice, it's fascinating.