What is the central metaphor, or comparison, in the last stanza? the railway train

The whole poem is really a metaphor in which the locomotive is compared to a kind of super- horse which is "docile and omnipotent". Miss Dickinson sees that man has harnessed the power of this huge locomotive and made it as obedient to his will as a thoroughbred race horse.

Explanation:

substitute y = 2x - 4 to x + 2y = 12:

x + 2(2x - 4) = 12     use distributive property

x + (2)(2x) + () = 12

x + 4x - 8 = 12     add 8 to both sides

5x = 20     divide both sides by 5

x = 4

put the value of x to the first equation:

y = 2(4) - 4

y = 8 - 4

y = 4