How are the religious usages of mainline and evangelical related to their general nonreligious meanings?

I have some but not much idea about the differences between Mainline and Evangelical. I looked up the two words in dictionary, but I am wondering what relations are between their general non-religious meanings to their religious usages? For example, I can't tell how the meanings in https://www.merriam-webster.com/dictionary/mainline relate to its religious usage.