Photo of the California Zephyr at Glenwood Springs, Colorado.  The town's Hotel Denver is seen in the background.
Hotel Denver-Robert Bishop, Denver-photographer
Not mailed.  The photographer's address has a zone number rather than Zip Code in it.  They were used in large US cities before the advent of Zip Codes circa 1963-1964, so before 1964.
<p><a rel="nofollow" class="external text" href="http://www.ebay.com/itm/Postcard-Hotel-Denver-Vista-Dome-Zephyr-Train-Glenwood-Springs-Colorado-/360750561755?pt=LH_DefaultDomain_0&amp;hash=item53fe68b9db&amp;nma=true&amp;si=jutes3RMBFnw%252BMWnEervC03rGi0%253D&amp;orig_cvip=true&amp;rt=nc&amp;_trksid=p2047675.l2557">eBay item</a>
<a rel="nofollow" class="external text" href="http://thmb.inkfrog.com/thumbn/woody64/bs_ara1.jpg">front</a>
</p>
<a rel="nofollow" class="external text" href="http://thmb.inkfrog.com/thumbn/woody64/bs_ara2.jpg">back</a>