And though I may not guess the kind— Correctly—yet to me A piercing Comfort it affords In passing Calvary—
Translation? Allow Shmoop: the speaker is saying here that even though she can only know her own sorrow, and can't possible guess what's grieving everyone else, she still feels comforted by the fact that others are suffering like her.
Biblical allusion alert. The speaker mentions "Calvary," the site of Jesus' crucifixion in the Bible.
What's that all about? Well, passing by it seems to have something to do with the comfort she feels in witnessing others' suffering. But not in a completely creepy way, of course.
In fact, she could be alluding to the common Christian belief that Christ suffered for humanity's benefit—that his crucifixion helped lead to salvation.
To note the fashions—of the Cross— And how they're mostly worn— Still fascinated to presume That Some—are like My Own—
Metaphor alert. The speaker refers to kinds of grief as "fashions," or clothes that people wear.
As she passes by Calvary hill (where Jesus was crucified), the speaker notes the suffering of those around her, fascinated that she seems to think that their grief might be like her own.
So she ends the poem just as she began it, "still fascinated" by the suffering of others, and still holding on to the notion that some people's suffering matches her own.
Here, she seems to be using the crucifixion as a symbol of everyone's pain and suffering, which makes sense, given the Christian beliefs about it. The idea here is that the suffering of others gives her a "piercing Comfort," or solace, in being able to recognize the grief that others experience, and to hope that maybe hers is the same.