"Mainline" is a term coined by the media to reference non-Evangelical, non-Fundamentalist and non-Catholic denominations.
Mainline Protestant denominations include Presbyterian, United Methodist and the Episcopal Church. The leadership of these bodies is almost completely liberal, far moreso than the people in the pews.
That is not to say that everyone who attends these churches agrees with the leadership. I have a friend who is a Prebyterian minister who is deeply devout and is quite conservative.
ok, that makes sense kind of I think mainline is to mislead people i suppose.