Mainline refers to the traditional nationally-based (as opposed to more regionally oriented) Protestant churches, almost all of which accepted the 'higher criticism' in the 19th and early 20th centuries, did not reject science where it clashed with the Bible, and most of which, to one degree or another, were involved in the 'social gospel' liberalism in the 20th century. They were, and to a large extent are, the churches of educatated opinion in the United States. The most generally agreed "mainline" Protestant Churches are: the Episcopal Church in America, United Church of Christ (Congregational in New England), Presbyterian Church USA and the Evangelical Lutheran Church of America. Sometimes the United Methodist Church is considered mainline, sometimes not, likewise the northern Baptists. If there are any Dutch Reformed Churches still around (mostly NY), they're mainline. The Southern Baptists are not mainline, nor are the Missouri Synod Lutherans. If the Unitarians (19th Century derivative from Congregational) are considered Christians, they'd be mainline.
Everything else is not 'mainline': Assembly of God, Disciples of Christ, Covenant Church, Pentacostals, etc., etc.