SEPTA is soliciting proposals from engineering firms to do the final design work beginning this fall for its $2 billion plan to extend light-rail service to King of Prussia.

That’s another step among many in the competitive federal Capital Investment Grant program for big transit projects — but a pivotal one.

Final design is the last big bureaucratic hurdle before the transit authority finds out whether it will win one of those construction grants, said Anna Hooven, SEPTA’s project director for KOP Rail.

“It does feel like there’s a lot of momentum around this right now,” Hooven said in an interview.

The project would extend the Norristown High Speed Line four miles to King of Prussia, with five stations, ending with a stop near the Valley Forge Casino Resort, which has a convention center. It would link the second largest employment center in the region to its largest, Philadelphia, via the 69th Street Transportation Center.

Construction costs are projected at about $1.6 billion, though officials are considering a revision of the estimate for inflation. The rest of the estimated cost is for planning and engineering design. KOP rail has been discussed and debated for years.

SEPTA pledged $390 million toward the project in its recently enacted 12-year capital spending plan.

Some work could start in 2024, followed by construction of the elevated guideway, the bed for the tracks, beginning in 2025, Hooven said. “To me, it seems like it’s just around the corner.”

An industry forum for potential contractors is scheduled for June 27, with the location to be announced.

Trains are projected to begin carrying passengers on the extension in 2027, according to the Federal Transit Administration.

First, SEPTA has to win federal funding, a decision that is expected to be made sometime next year.

“It’s unique that SEPTA has to make the financial commitment leading up until that [point] not really knowing if we’ll be successful or not,” Hooven said.

More money than usual will be available. The FTA increased the budget for the grant program to $4.45 billion a year — from $2.3 billion — amid the relative gold rush of the first increase in federal infrastructure spending in decades.