<p>The San Martin stop is in Retiro, one of BA's ritziest neighborhoods.  I have no idea why the letter 'B' is highlit; only the 'C' line stops here.
</p><p>(Update, 12/7/2014: The letter 'B' is highlit 'cause it's out of service.  Thanks, Ricardo Barreiro, for letting me know.)
</p>
I worried needlessly in the states about how I'd nab a SUBE IC card. (sube.gob.ar)  True, they don't sell 'em at the airport -- and I'm sure other tourists would be grateful if they did -- but I had no trouble buying one at a kiosk down the stairs in this station.
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/36217981@N02">Tim Adams</a> from San Francisco
CC BY 2.0
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/36217981@N02/15318579914/">Subte metro entrance in Buenos Aires, Argentina</a>