"His Jacket Was Unbuttoned, and On the Front of His White Shirt, About Eight Inches Apart, There Were Two Blood Spots About the Size of Fifty-cent Pieces"

"The Body in Roomette 9."
By Howard Rigsby (1909-75) and Luraine Rigsby (?-?).
First appearance: Collier's, November 9, 1956.
Short story (18 pages).
Online at UNZ: Start (HERE) and finish (HERE; scroll down 

to page 78).

"The trip to New York was a bonus from the boss to the employ-ees of Willets and Company . . . Just one big happy family, they seemed, until a smoldering hatred flared up into murder, and terror took over the train."

Jeff Byrne, our first-person narrator, tells us what he does for a living, and it isn't private detective:

   "Have you heard of show trains? Well, they're the rage these days. Pay your money down, and you get entertainment in a great big package; a trip to New York on a special train and tickets to some hit Broadway shows, plus hotel accommodations, meals, sight-seeing, culture—even romance, maybe. My name is Jeff Byrne, and I'm a promoter of such trains. Making up one of these package trips always presents plenty of headaches and trouble, 
but I never really knew what trouble was until I got murder mixed in with the package."

The murder victim, it eventuates, is the head of a successful company that Jeff can't help characterizing as being replete with people who, while they "worked together" also "brought with them all their rivalries, jealousies and tensions." In Jeff's wry assessment, "What goes on in this company would make fine material for a soap opera"—which the man found in Roomette Nine would probably agree with if two bullets hadn't severely curtailed his amo-rous activities.

Jeff's involvement with the crime is remote at first; it's only after he's experienced love at first sight with the prime suspect that he becomes, to the annoyance of the official police, an amateur sleuth (very amateur) convinced that his inamorata is not only innocent but also being neatly framed with every piece of circumstantial evidence that turns up.

Because of this murder Jeff, a man whose profession is to help keep people entertained, can hardly believe what's happened to him in the last twenty-four hours, and at one point he pauses briefly to reflect: "Here I was, I thought, ghosting around searching luggage and knocking people down—and my only excuse was that the night before I had walked into 
that club car and fallen in love."

Resources:
- The French Wikipedia has an entry about Howard Rigsby (HERE).
- According to the IMDb, a Western novel by Rigsby (writing as Vechel Howard) served as the basis for a movie (HERE) and one of his stories inspired a TV episode (HERE); also see FictionMags for a list of his short fiction. As for Luraine, this appears to be her only story credit.

The bottom line: "People more often kill those they love than those they hate. Possibly because only the people you love can really make life unendurable to you."
― Agatha Christie