Telephone mainlines are telephone lines connecting a customer's equipment to the public switched telephone network.
I would call it a traditional spiritual song. Ry Cooder has several good versions on youtube
Wash Hobson Sr. from Rockford IL.
It indicates that it is a spur off of the mainline highway (Interstate 76). In this case it does not reconnect to the mainline but to a different mainline (Interstate 95).
It means the death of Jesus.
The term "mainline merchandising" is a reference to the product flow. Usually the main line begins with the production of the product and ends with the distribution of the product.
I guess it's about Jesus
Mainline Steam was created in 1988.
Mainline Airways ended in 2003.
Midland Mainline ended in 2007.
Midland Mainline was created in 1997.
Mainline Publications was created in 1953.
Though Neutral Milk Hotel does have a song about Jesus Christ, they are not Christian. On a live version of the song, Jeff Mangum states before he plays "Here's a song about Jesus. I mean, I'm just as confused as everybody else... but it's always nice to sing a song about Jesus."