The builder of the Foothill Gold Line’s next segment extending from Glendora to Montclair is accusing Metrolink, the regional commuter rail that will share the route, of making outrageous demands that would increase the project’s cost by millions of dollars and delay completion by two years.

According to a report written by Gold Line Construction Authority CEO Habib Balian, Metrolink’s operators are suddenly asking for a $2.7 million design review of the shovel-ready project.

The review requested by the Southern California Regional Rail Authority would cost nearly one-third the total price of the design already completed. Balian called the amount “engorged.”

Metrolink also asked the Gold Line Construction Authority to add another four to six bridges that would separate the light-rail train from the street level in an area near Claremont, said Balian during an interview Tuesday.

Adding several unplanned, concrete train bridges could take another two years of study, design and engineering, he said, making a nine-year project stretch to 11 years.

“The big thing is the grade separations. Those are big structures and are new elements offered up very late in the planning of this project,” Balian said. “That could easily take two years.”

The 12.3-mile extension would pass through six cities and is scheduled to start construction in October on utility relocation work. Any changes to the design of stations, track alignment or new bridges must be incorporated into the contract, which is presently being readied for an early 2018 bid process, wrote Doug Tessitor, chairman of the Construction Authority board in a statement.

If the Gold Line is forced to add more elements, that will lead to cost overruns and mid-project stoppages, he said. The complicated project requiring shifting freight tracks and adding light-rail tracks and overhead electric wires is scheduled for completion in early 2026.

“These kinds of delays are an effort to throw a wrench into the project,” Tessitor said during an interview.

Tessitor said the requests made by Metrolink and its CEO Art Leahy could add multiple millions of dollars to an already expensive light-rail extension that the Construction Authority estimates will cost $1.5 billion.

In Balian’s report, he said L.A. Metro, which operates the Gold Line, submitted 195 comments to engineering plan for the project.

Metrolink, Balian said, submitted 1,173 comments. He said the flood of comments was an effort at “largely redesigning the project.”

“Metrolink and Art Leahy are doing whatever they can to try and delay the project,” Tessitor said.

A Metrolink spokesman said Leahy was not available to comment on the report or Balian’s allegations.

In an emailed statement, Metrolink Board Chairman Andrew Kotyuk said the commuter rail’s staff “raised key issues that are critical to our safe and efficient coexistence (with the Metro Gold Line).”

“With over 20 trains being planned during peak hours through the cities where both the Gold Line extension and Metrolink exists, it is critical that we have a plan to address safety and traffic issues that construction and the new frequency of service may cause,” Kotyuk said in the statement.

Balian said the Gold Line had addressed “all these issues.” He said he believed the current design was safe.

“We’ve been working on them with Metrolink,” he said. “They’ve been at the table.”

The fight comes amid concern from Metrolink about a 7.6 percent drop in ridership on the San Bernardino Line, the train system’s most used. A Metro report also showed riders were down 25 percent at Metrolink’s Covina station.

Flagging ridership was attributed to the popularity of the Foothill Gold Line beginning in Azusa, which has experienced overflowing parking and crowded trains.

The Gold Line is also cheaper — a one-way ride from Azusa to Union Station costs $1.75, with two hours of free transfers. Riding Metrolink from Covina to Union Station costs $7.75.

With the Glendora-to-Montclair extension, competition between the two transit lines would intensify.

The Foothill Gold Line will become a second option for commuters going to downtown Los Angeles by rail and will have connections at Metrolink stations in Pomona, Claremont and Montclair.

How the two lines can complement each other will be the subject of a motion by four Metro board members to be heard at the Metro board meeting Thursday. Supervisors Kathryn Barger and Hilda Solis, plus Duarte Mayor John Fasana and Glendale Councilman Ara Najarian, are asking for a study of ridership patterns and whether some Metrolink stations should be eliminated once the Gold Line is operating.

A related study will evaluate of whether to keep the Claremont Metrolink Station and add a Gold Line station instead, or to eliminate the Metrolink station altogether. The decision must be made in 60 days, leaving enough time to proceed on schedule with the Gold Line Construction Authority’s bid process, Tessitor said.

Having the Gold Line as an option in the eastern end of Los Angeles County and in western San Bernardino County would add a mix of transportation services and attract new mass transit riders, according to four board members’ motion.

“However, three co-located Metrolink and Gold Line Stations — North Pomona, Claremont and Montclair — within a couple miles of each other may also prove to be redundant and therefore unwarranted,” according to their motion.