A study of a possible passenger rail service between Tucson and Phoenix has taken another step forward, with a federal agency officially selecting a preferred route.

That corridor, known as the “yellow corridor alternative,” would take a train along existing Arizona Department of Transportation and Union Pacific rights of way, according to a recently finalized record of decision from the Federal Railroad Administration.

Between Tucson and Eloy, the passenger train would parallel Interstate 10, and then follow UP’s corridor north from Eloy and eventually on to downtown Phoenix, according to the final Tier 1 Environmental Impact Statement, which was completed by the railroad administration and ADOT in December.

The railroad administration considered that route “more cost efficient and better performing” than the “orange” alternative also being considered. It would also “provide shorter trip times to a larger total number of riders” and “lower” environmental impacts, including effects on wildlife corridors, views and water resources, according to the record of decision. ADOT had also previously recommended the yellow option in a previous draft environmental review, and the most recent railroad administration decision approved that recommendation.

However, elements of the orange route would still be considered in the next stage of assessment to avoid impacts on historical and environmental resources. No funding has been secured for the project or for the next round of environmental impact assessment.

“Any next steps, including whether to fund the Tier 2 environmental studies, which are needed to advance the selected corridor alternative and to determine any schedule and funding for design and construction of the project, would be up to policymakers and the public,” according to an ADOT statement provided by a spokeswoman.

ADOT believes that any funding and construction of the project would happen “incrementally,” according to the record of decision.

The yellow corridor would cost an estimated $4.5 billion in 2013 dollars, versus $7.6 billion for the orange. The former would also serve more total daily passengers — over 20,000 — though fewer intercity passengers than the latter.

If constructed, the rail service would provide both commuter and intercity service. Priscilla Cornelio, director of Pima County’s department of transportation, said she was pleased to see both services prioritized.

A copy of the record of decision and related documents can be found at tinyurl.com/hmj6q67.

Contact: mwoodhouse@tucson.com or 573-4235. On Twitter: @murphywoodhouse