What kind of name is "Underground Railroad"?

Solution 1:

Sounds like a kenning to me. A kenning is a compound, usually consisting of 2 nouns, which metaphorically describe something or someone.

Examples:

"Missile toes"---a marathon winner. "I'd never participate in a marathon; I'm no missile toes."

"Gutsquatters"---intestinal parasites. "Those darn gutsquatters I picked up in South America have been plaguing me."

"Word pruner"---editor. "After a word pruner finishes checking over a manuscript, as much as half of it might be gone."

(http://examples.yourdictionary.com/examples-of-kenning.html)

Solution 2:

Figurative might be what you are looking for.

The figuratively named "underground railroad" helped to smuggle slaves to free land.

Solution 3:

It's a linguistic device originating in Sanskrit and also found in Old English and Old Norse poetry.

It's a compound expression known as Bahuvrihi, similar to an idiomatic expression but here the literal meanings of the component words are also significant.

Bahuvrihi (WP)

A bahuvrihi compound (from Sanskrit: बहुव्रीहि, literally meaning "much rice" but denoting a rich man) is a type of compound in Sanskrit grammar, that denotes a referent by specifying a certain characteristic or quality the referent possesses.
Examples

• "Houndstooth", a woven fabric with a patterns resembling dog's teeth: "She's wearing houndstooth."
• "Old money", members from established upper-class who have usually inherited their wealth: "He's definitely old money."
• "Bluestocking", an educated, intellectual or artistically accomplished woman: "Auntie Maud will never marry; she's a bluestocking."