«The Last Express» by Baynard Kendrick

A puzzling murder has a blind detective investigating the New York City subway in this Golden Age mystery series opener.
Following the loss of his sight in World War I, ex–intelligence officer Capt. Duncan Maclain honed his other senses and became one of the most successful and well-known private investigators in the Big Apple. Kind and sympathetic, he’s eager to help anyone who visits his office.
Evelyn Zarinka’s at the end of her rope, and at her fiancé’s suggestion, she goes to see his friend Maclain for help. For the past three months, Evelyn has noticed a startling change in her brother Paul, assistant district attorney for New York City’s Homicide Division. First it was the strange phone calls; then he bought a gun and started drinking heavily. And it’s only gotten worse. After speaking with Evelyn, Maclain is certain Paul’s in trouble.
But the detective is too late to help him. Paul’s car explodes, killing the assistant DA. The only clues are two caged white mice left in the back of the car—and Paul’s dying words: “Sea Beach Subway—the last express!” Now, with his seeing-eye dog, Maclain finds himself in Manhattan’s seedy underbelly—and its literal underground—on the hunt for a killer willing to do anything to throw the investigation off the rails . . .
Baynard Kendrick was the first American to enlist in the Canadian Army during World War I. While in London, he met a blind English soldier whose observational skills inspired the character of Capt. Duncan Maclain. Kendrick was also a founding member of the Mystery Writers of America and winner of the organization’s Grand Master Award.