According to the caption, this picture was taken in August 1917, it seems that that month was a rainy one too! According to the caption, these men are brought to the front in a truck. As they don’t seem to have any equipment, helmets nor rifles, I think that these men are just going back to their camp after a journey to the nearest French village of Lorraine.
By the way, do you remember Patton’s words about Lorraine? This was written during WW2, but he fought in Lorraine during the Great War too:
“I hope that in the final settlement of the war, you insist that the Germans retain Lorraine, because I can imagine no greater burden than to the owner of this nasty country where it rains every day and where the whole worth of the people consists in assorted manure piles” (letter to the Secretary of War Henry L. Stimson)