The nail with the PK on it is called a PK nail named after the Parker-Kalon Company. The nails were used for surveying to mark a precise location.
Your other nail with the 20 on it is a tie date nail. These were installed in new railroad ties. The number indicates the year of install, in this case 1920. Many different forms of tie preservation was being experimented with early on so they would mark the ties with the nails and keep a log of how long they lasted and how well they held up.
Your last two items appear to be magazine advertisements for the American Locomotive Co. (Alco) and the Association of American Railroads.
The Parker-Kalon company commissioned one of the rarest Lionel postwar 6464 boxcar variants. It was molded in gray and had decals which said "Another carload of Parker-Kalon screws"
