I heard Mylon LeFevre relate in an interview that the idea for the song came when his son said that a thunderstorm sounded like "trains up in the sky." I don't know what meaning Mylon meant, but the words are reminiscent of the scripture Isaiah 6:1, "In the year King Uzziah died, I saw also the Lord sitting upon a throne, high and lifted up, and his train filled the temple." Yes, it's a different kind of train, but I think one led to the other. This is further supported by the lyrics, "I want to see the Holy; I want to see the Holy one: I want to see the Holy One of Israel." The phrase "Holy One of Israel" is used in the book of Isaiah seven times (at least), and if I remember correctly, it is a phrase used exclusively in the book of Isaiah. The "train" that fills the temple is the length of a king's royal garments that trails him when he walks. Supposedly, the longer the train, the greater the king. God's (The Holy One of Israel) train is so long that it fills the temple. He is the greatest of kings--the King of Kings.