Can anyone recommend some detailed and well researched sources on the development of the knuckle coupler and its adoption? Good articles or books?

There is one book on the subject, its by National Steel Castings.

See if you can search and locate it, and if not, ill try to post a link in a few days.

John White's books, the American Passenger Car and the American Freight car would be a good place to look. Also the MCB Proceedings, starting about 1894 to about 1915 would be right from the horses mouth. The reading is a bit dry.

The modern knuckle coupler has its origins with the patent of Eli H. Janney granted on April 29,
1873. In 1877 he associated himself with the firm of McConway & Torley which produced and improved
the coupler. Mr. Janney continued to patent improvements to the coupler as late as 1912 – the year of his
death. In time the Master Car Builders Association adopted the term “vertical plane coupler” to identify it
in a generic manner. But, the M.C.B. was reluctant to adopt a design whose mating face patent was
controlled by one firm; in 1888 it reached an agreement with McConway & Torley that placed the mating
face design in public domain. The Master Car Builders Association then adopted the design as the
“M.C.B. Coupler.” During this period there were many other variants and improvements patented by
others. By the end of 1899 the M.C.B. had approved forty-nine manufacturers of the “vertical plane
coupler” that would work together.

The United States in 1893 enacted “Safety Appliance Act” and it became law on March 2, 1893. This
provided for application of automatic couplers, automatic brakes and common locations of grab irons on
railroad cars. The initial change over date was January 1, 1898, but this was extended to July 1, 1900.
The railroads were opposed to the required changes and the costs of doing these.

The law stated, “On January 1, 1898, it will be unlawful for a common carrier used for interstate
commerce to haul or permit to be hauled any car that is not equipped with couplers coupling
automatically by impact and which can be uncoupled without a man going between the ends of the cars.”

I would like to see such a book too. I know that there were many coupler variations throughout the link and pin era. I used to think that once the Janney was introduced, the coupler grew larger over time, but the principle remained the same. Now, I am not so sure about that.

The first time anyone would notice the knuckle pin was missing was when the car was uncoupled and the knuckle fell out onto the track.

When the knuckle pin starts to get pulling and buffing stress it cracks and the pin will fall out of the bottom of the coupler. The head of the pin stays in place, on top of the coupler, to hid the danger. That is why you'll easily find a few headless pins laying in every rail yard.

With the pin missing, but the pinhead still in place, when you swing the knuckle open, it over rotates and comes out of the head just far enough to land on your foot. This occurrence breaks more feet and toes than any other railroad injury.