What is the difference between mainline Baptist and evangelical Baptist?
As far as I know there is very little difference in doctrine except most Baptist churches are affiliated with the Southern Baptist Convention (SBC) but there are several other much smaller conventions in the US. A somewhat more liberal (for lack of a better description) is the American Baptist Church USA, and it's the only one I can think of offhand. My family and I were members of an independent Baptist church for many years. But when moved to a small town in north GA the only Baptist churches anywhere within reasonable driving distance are SBC, but the basic doctrines and worship are virtually identical to that of the SBC.
One exception is that a certain % of tithes and free will offerings to local churches go to the SBC mission board to support SBC seminaries and and thousands of foreign missionaries serving all around the globe. The large independent Baptist church that we had attended for decades before moving sent out and supported it's own foreign missionaries. Around 30 IIRC, and that includes my own daughter.
I have heard that the American Baptist doctrines and practices are similar to those of the SBC on paper, but in actual practice the ABCUSA is considerably more liberal (again for lack of a better description).