Well, I'm glad that my speculation that a railgun, even on Earth, is a
useful step in obtaining very cheap access to space seems to have some
validity, as the same idea is being studied by those who should know
what they're doing:

http://www.nasa.gov/topics/technolog...tallaunch.html

This news item dates from September.

The scheme proposed, though, appears to involve the railgun-assisted
launch of an aircraft, which will then launch a rocket from a high
altitude. It is not clear to me that a railgun will really benefit an
aircraft that much, since aircraft are already much more economical
vehicles than space rockets. But then, a railgun that accelerates its
payload to 600 miles per hour is much more modest than one that
accelerates it to 16,000 miles per hour.