I took this at the time Southwest was ramping up services at the East Coast airport.  I managed to book a flight on the inaugural flight between BWI and Midway, I think.  I recall it being the whale-logo aircraft.
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/130275500@N02">G B_NZ</a>
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<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/130275500@N02/22015628110/">Southwest B737-200 at BWI</a>