The Underground Railroad was neither underground nor a railroad. It got its name because its activities had to be carried out in secret, using darkness or disguise, and because railway terms were used by those involved with system to describe how it worked.
Hydrolyzed collagen is collagen that is derived from bovine bone and cartilage. Usually, the bone is crushed, ground, defatted, soaked in acid to remove the calcium, soaked again to break the collagen bonds and then dehydrated.
According to Wikipedia, hydrolyzed protein is protein that has been hydrolyzed or broken down into its component amino acids. As the name implies, the process of hydrolysis will include adding water to separate the amino acids. The most common hydrolyzed food component in dog food is yeast. Other hydrolyzed proteins include hydrolyzed chicken and hydrolyzed soy. Depending on the food, hydrolyzed proteins are used for different purposes. Hydrolyzed yeast is used as both a flavor enhancer and a as a hypoallergenic alternative for yeast protein.