single file, in
Also, in Indian file. Aligned one behind the other, as in We have to bike in single file here, or The children were told to march in Indian file. Both usages are associated with military formations; the first term was first recorded in 1670; the variant, alluding to the usual marching order of Native Americans, was first recorded in 1758.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.