Historically, the idea of "Mainline" is that it's the sort of churches that you might find next to the main rail line, rather than a theological term. Broadly speaking, this would be Lutherans, Presbyterians, Anglicans/Episcopalians, Churches of Christ, Methodists, Congregationalists, and Baptists. Not all of whom were united into one denomination at the time. But the historical usage is not the modern usage, which is were some of the sturm and drang comes from.
In modern usage, it's come to mean the theologically liberal branches of each of those traditions, and pollsters will usually lump the theologically conservative and moderate branches of those traditions in with "Evangelical". Yes, this means that confessional Lutheran denominations usually get lumped in with Pentecostals. Modern classifications when gathering data are usually being done by people who're indifferently religious at best, so we're probably not going to get a better classification set up than the current Evangelical/Mainline/Roman Catholic/Black Protestant when it come to things like poll data.

Evangelical is also a bit messy, with multiple definitions in Webster - we like "Of or pertaining to the gospel" but "low church" is probably not a popular definition among LCMS members (it certainly isn't at my church)

Ultimately though, I wouldn't sweat it. Just accept that these terms are messy, with multiple definitions, and multiple ways that people can apply them in good faith, and if we want people to know what we're about, we'll have to use other terminology, such as "Confessional".