Mind the gap! Sorry London underground lingo, you'll understand if you've ever been.
Why doesn't it accept "i am at an underground station" and suggests "I am at a underground station" instead? Surely you would use "an" before a word like "underground".
Someone wrote 'at [a/the] [subway/metro/tube/underground] station' without double checking. Of course you're right, added now.
In the US, train is used more than subway. Subway is usually used when referring specifically to New York