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Consensus seems to be that, as a US citizen, for travel into the US from Canada, the two cards are virtually interchangeable. That also includes using the expedited security line in Toronto when heading to St. Louis, for example (pre-clearance). Please correct me if I'm wrong.
Answers are contradictory, however, when the question is whether a Nexus card held by a US citizen allows expedited entry into the US from the UK or Germany, for example, as the Global Entry card does. Could someone clarify that?