In Phillis Wheatley's famous poem in praise of George Washington "To his Excellency George Washington," Wheatley uses a neoclassical style with classical allusions (e.g., "The goddess comes, she moves divinely fair, / Olive and laurel bind her golden hair") to describe Washington leading the nation in war. While America is "Columbia" (e.g., "When Gallic powers Columbia’s fury found"), Washington's army is metaphorically compared to a "warrior's train" (line 20), a "martial band" (line 26), and "Columbia's arm" (line 34).
The comparative metaphor of a warrior's train brings up images of Medieval warriors fighting for truth and righteousness arrayed in ranks and rows behind the valiant Lord or Knight or Sovereign King leading them forward. A "train" in this sense is the military formation of troops in rank and file. Washington's "martial band" is a military allusion to a united group dedicated to one purpose (i.e., band), while "martial" refers to that which is associated with armed forces.
In calling America "Columbia," Wheatley echoes the female name for the United States that alludes to the name of Christopher Columbus and that was used for the British American colonies as early as 1730 (Online Etymology Dictionary at Dictionary.com). So "Columbia's arm" signifies the military strength and power of the nation of the United States.