Extend Wireless Network .6 miles

I need to extend my wireless network about 0.6 miles away. I have an
okay view from on top of my building to the place that I want to send
the signal too.

There are some obstacles in the way though. There is a path of
neverending trees, I think I can get past them with a strong enough
signal. About 0.3 miles away there is a station for an elevated train,
so I don't know if the concrete structure will be a problem, its about
15 feet high, or if the metal of the train will bounce the signal. I
am also worried about WLANs in the area and cordless phones, I think I
can decrease that interference by changing my WLAN frequency to one
that is not used.

I am looking at a 2.4 GHz 18dbi 30 degree directional antenna, would
prefer a smaller degree, but no biggie. Do you think that a single
18dbi directional antenna would reach my destination?

A 802.11a antenna would work also, but I have not seen one that would
reach far enough.

Any suggestions on this would be great. Thanks.