TransCanada said it is "considering all options" after Obama denied the project Friday.
Officials believe the pipeline will be approved, if not by Obama, then by his successor.
"I remain very confident" the pipeline will be approved, CEO Russ Girling said.
TransCanada CEO Russ Girling said Obama's rejection wouldn't stop pipeline push.
"The demand continues to be there and support continues to be there," Russ Girling said.
Oil began flowing through the pipeline's southern leg on Wednesday.
"If we need to bridge with rail, we will bridge," TransCanada's CEO said.