Each company had one wagoner. Their job, as the name implies, was to drive the wagon carrying company equipment as they moved from place to place. When large armies were moving there would be hundreds of wagons and wagoners carrying the supplies critical to the army's success. Because of this the wagoners were frequently targets of attacks. There are many instances in which wagoners fought in battles and played critical roles in the success of entire armies.
Most of the wagoners of the 13th Maine were were older men ranging in age from 35 to 51 years old at enlistment. They were generally larger men. Two of the wagoners of the 13th Maine were over six feet tall.
Most of these men were teamsters in civilian life and, in fact, they functioned more like civilians. Typically, they did not wear regular uniforms and they wore broad brimmed hats.
According to the general orders from the War Department the wagoners were to receive the pay and allowances of corporals of cavalry.
The man in the picture was probably either Albion Grant or Eli Aldrich of Co. K. Notice that he is wearing civilian clothes and stands taller than the other men near him.