"The Four Steps"

"Now, i ask you to take the Sacred Steps, in union with the Eternal Engine... These Four Steps represent the path humanity has taken. From Grass, to Ice, from Ice, to Steel. And from Steel to the Lights, of the Eternal Engine." - Mr. Wilford
The Four Steps appears to be a marital tradition passengers make when they are married inside Snowpiercer and Big Alice. LJ Folger and Ozweiller are the only ones shown in the series to have experienced the Four Steps (and the marriage ceremony in its entirety).