Here's the original poem, in context, from Works, Volume 23: Letters by Robert Louis Stevenson (New York: Charles Scribner's Sons, 1899), page 167:

To Sidney Colvin

[In The Emigrant Train
from New York To San Francisco,
August, 1879.]

Dear Colvin,— I am in the cars between Pittsburgh and Chicago, just now bowling through Ohio. I am taking charge of a kid, whose mother is asleep, with one eye, while I write you this with the other. I reached N. Y. Sunday night; and by five o'clock Monday was under way for the West. It is now about ten on Wednesday morning, so I have already been about forty hours in the cars. It is impossible to lie down in them, which must end by being very wearying.

I had no idea how easy it was to commit suicide. There seems nothing left of me; I died a while ago; I do not know who it is that is travelling.

Of where or how, I nothing know;
      And why, I do not care;
      Enough if, even so,
My travelling eyes, my travelling mind can go
By flood and field and hill, by wood and meadow fair,
Beside the Susquehanna and along the Delaware.

I think, I hope, I dream no more
      The dreams of otherwhere,
The cherished thoughts of yore;
I have been changed from what I was before;
And drunk too deep perchance the lotus of the air
Beside the Susquehanna and along the Delaware.

Unweary God me yet shall bring
      To lands of brighter air,
      Where I, now half a king,
Shall with enfranchised spirit loudlier sing,
And wear a bolder front than that which now I wear
Beside the Susquehanna and along the Delaware.

Exit Muse, hurried by child's games. . . .

Have at you again, being now well through Indiana. In America you eat better than anywhere else: fact. The food is heavenly.

No man is any use until he has dared everything; I feel just now as if I had, and so might become a man. "If ye have faith like a grain of mustard-seed." That is so true! Just now I have faith as big as a cigar-case; I will not say die, and do not fear man nor fortune.

R. L. S.