What does "mainline" stand for here?

The Mainline Protestant churches (also called mainstream American Protestant[1] and "oldline Protestant" are a group of Protestant churches in the United States that contrast in history and practice with evangelical, fundamentalist, and charismatic Protestant denominations. Mainline Protestants were a majority of all Christians in the United States until the mid-20th century, but now constitute a minority among Protestants. Mainline churches include the United Methodist Church (UMC), the Evangelical Lutheran Church in America (ELCA), the Presbyterian Church (U.S.A.) (PCUSA), the Episcopal Church, the American Baptist Churches, the United Church of Christ (Congregationalist), the Disciples of Christ, Reformed Church in America, and Hicksite Quakers, among others.

Once I saw this guy on a bridge about to jump. I said, "Don't do it!" He said, "Nobody loves me." I said, "God loves you. Do you believe in God?"

He said, "Yes."

I said, "Are you a Christian or a Jew?"

He said, "A Christian." I said, "Me, too!

Protestant or Catholic?" He said, "Protestant." I said, "Me, too!

What franchise?" He said, "Baptist." I said, "Me, too!

Northern Baptist or Southern Baptist?" He said, "Northern Baptist." I said, "Me, too!

Northern Conservative Baptist or Northern Liberal Baptist?" He said, "Northern Conservative Baptist." I said, "Me, too!

Northern Conservative Baptist Great Lakes Region, or Northern Conservative Baptist Eastern Region?" He said, "Northern Conservative Baptist Great Lakes Region." I said, "Me, too!"

Northern Conservative Baptist Great Lakes Region Council of 1879, or Northern Conservative Baptist Great Lakes Region Council of 1912?" He said, "Northern Conservative Baptist Great Lakes Region Council of 1912."

I said, "Die, heretic!" And I pushed him over.