Proper Noun:
* When referring to the specific Subway restaurant chain, it's a proper noun and should be capitalized: "I had lunch at Subway today."
Common Noun:
* When referring to an underground railway system in general, it's a common noun and doesn't need to be capitalized: "The city has a new subway line."
Here's a breakdown:
* Proper noun: A specific name of a person, place, or thing.
* Common noun: A general name for a person, place, or thing.
So, if you're talking about the Subway restaurant chain, it's a proper noun. If you're talking about an underground railway system in general, it's a common noun.