AACHurdles

hur⋅dle
 / ˈhɜrdl /    [  hur -dl ]  noun, verb, -dled, -dling. –noun [] ​
 * Definition:** Any of various vertical barriers, as a hedge,​ low wall, or section of fence, over which horses must jump in certain types of turf races, as a steeplechase, but esp. an artificial barrier.

"Hurdles and guns and railings, as the boulders heave," so the speaker is using hurdles as a barrier that he has to over come in this journey. I find that the speaker want us to vizulaize the hurdles as a obsitcle in our life that we have to over come, but in the poem hes talking about the hurdles the guns and the railings which can give people another thought.