For an urban rail system running largely below ground, “subway” is mainly American English while “the underground” is British English. “Subway” also has a separate British pedestrian-passage sense.
ReviewedEvidence2 sourcesSectionEnglish Usage
Quick answer
Use “subway” for the urban rail system in US English and “the underground” in UK English; remember that British “subway” can instead mean a pedestrian underpass.
Key details
Confusion Familyregional transport vocabulary with polysemy
Rule FamilyBritish and American vocabulary
Termssubway, underground
Variant ScopeUS “subway” corresponds to UK “the underground” for rail transit, but UK “subway” can mean a pedestrian passage under a road.
Further guidance
Core difference
The rail-system meanings are regional counterparts: US “subway,” UK “underground.”
Fast Test
If your audience is British and you mean trains, “the underground” is the safer general term.
Usage Boundary
Keep the British pedestrian-passage sense of “subway” separate from the rail-system comparison.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
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