It Is Better Farther On could be be based on the popular anonymous poem "A Song of Hope" also called "Farther On."

I hear it singing, sweetly singing,
Softly in an undertone;
Singing as if God had taught it,
"It is better farther on."


"It Is Better Farther On" is also a song found in The Evangelists' Songs of Praise, No. 2 By C. V. Strickland 1892

http://books.google.com/books?id=m-yRR4QMjC8C&pg=PA54&dq=%22It+Is+Better+Farther+On%22&lr=&ie=ISO-8859-1&output=html

Clearly there are similarities. The Carters clearly is a rewrite of an older existing song (or songs):

Check out the Carters 3rd verse with this found as early as 1836:

I. Say, young soldier, are you weary
Of the roughness of the way?
Does your heart begin to fail you,
And your vigor to decay?

http://books.google.com/books?id=XvB0ERB2iHIC&pg=PA16&dq=And+your+vigor+to+decay&ie=ISO-8859-1&output=html

It's almost exactly the Carter's third verse.

IT IS BETTER FARTHER ON- Carter Family

As we travel through the desert
Storms beset us by the way
But beyond the river Jordan
Lies a field of endless day


Farther on, still go farther
Count the milestones one by one
Jesus will forsake you never
It is better farther on

Oh my brother are you weary
Of the roughness and the way
Does your strength begin to fail you
And your vigor to decay

At my grave, o still be singing
Though you weep for one that's gone
Sing it as we once did sing it
It is better farther on