Dagny worships at the Church of Nat Taggart, and it's a religion with a bizarre messiah:
Many stories were whispered about him. It was said that in the wilderness of the Middle West, he murdered a state legislator who attempted to revoke a charter granted to him... but the charge could never be proved. (188.8.131.52)
This guy doesn't seem like a great role model. But Dagny sees him as more than some sort of legendary Old West figure; he represents hard work and achievement:
All that Dagny wanted of life was contained in the desire to hold her head as he did. (184.108.40.206)
Dagny wants to live the sort of bold and uncompromising life that Nat lived, and she finds inspiration in his struggles and his never-say-die attitude. She even communes with him during a meeting where the Board shuts down the John Galt Line she'd worked so hard to build:
"You had to bear it, too, generations ago – and it was just as hard for you, just as bad, but you did not let it stop you." (220.127.116.11)
Nat is a guiding presence in the book and ties in to the running themes of history, inspiring legends, and the importance of having people to look up to in life.