One day, in about five years, riders of the Metro Green Line will go from Norwalk to LAX. Folks riding the under-construction Crenshaw light-rail line also can connect to the airport. The lines will meet at the $7.3 million Airport Metro Connector 96th Street Transit Station where an airport people-mover will take them to the terminals.

“Hooray! It’s about time!” will undoubtedly be the refrains when the first LAX rail connection opens in 2021, a celebration promised repeatedly by Los Angeles Mayor Eric Garcetti during the campaign for Los Angeles County Metropolitan Transportation Authority (Metro)’s Measure M last fall, which passed with more than 71 percent of the vote.

But the Southern California Association of Governments says there’s still a big piece of the rails-to-airport puzzle missing. That missing link would be closed by connecting the two rail depots by an aerial train, underground subway, or dedicated busway. The plan is the subject of a public scoping meeting Wednesday at 6 p.m. at the Sproul Reception Center, 12239 Sproul St. in Norwalk.

Commuters on Metrolink, Metro’s heavy-rail cousin, come from Riverside and Orange County to the busy Norwalk/Santa Fe Springs Metrolink Station on Imperial Highway, located 2.8 miles away from the Green Line station, also in Norwalk.

Metrolink commuters are on a fixed route to Union Station in downtown Los Angeles. Hence, despite Garcetti’s promises, anyone who wants to reach LAX from the Inland Empire or Orange County must drive the 405 Freeway and fight some of the worst traffic in the Southland, not to mention getting swallowed up in airport traffic, parking costs and the infamous “horseshoe” of hell off Century Boulevard that allows one way in and out of the second-busiest airport in the United States.

“There is a gap between the Metrolink Station in Norwalk and the Green Line (Metro rail) Station also in Norwalk,” said Hasan Ikhrata, executive director of SCAG. “If you want people to take transit, it has to be connected.”

One could call SCAG’s project the Rail-to-LAX II.

While the first one should be operational around 2021, this small but significant light-rail rail extension connecting the two Norwalk train stations is in the conceptual phase, meaning Ikhrata predicts it would open in 10 years if funding can be arranged.

Metrolink supports any project that connects its commuters to more places, said Scott Johnson, spokesman.

Today, two commuter lines run through Norwalk but do not connect up with Metro’s Green Line. The Orange County Line, which runs from Oceanside to Union Station, sends 11 northbound trains and 10 southbound trains a day, he said. The 91/Perris Valley Line originates in Perris in Riverside County, runs parallel with the 91 Freeway through Corona, parts of Orange County, connecting the I.E. with Los Angeles County.

“It is not as easy for someone from Orange County to get to LAX unless this connector was finalized,” Johnson said.

“Right now, if I am a resident of Riverside and Orange County and I wanted to take Metrolink to get to LAX, my primary connecting point is LA Union Station,” Johnson said. “An additional connection at Norwalk prevents a person from going into downtown Los Angeles; it saves time.”

Whether Norwalk supports the project is unclear. Mayor Mike Mendez said he agrees the light-rail line should extend to the Metrolink station across town. But he is concerned about a raised train line being unsightly, or requiring eminent domain of properties. The issues arose in the 1990s when the idea first surfaced and was scuttled.

“Either way, the people were concerned,” he said, adding that the city would have to put up with construction noise and dust for the benefit of regional commuters more than its own residents.

Many of the rail connection issues for Southern California always come back to Union Station.

Most trains go through there, an historic depot on the northeast side of Los Angeles that’s not near work centers or urban condos. For example, civic center workers as well as bankers in the financial district must walk the halls of the building and travel down lengthy escalators to reach subways. Sometimes, they need to take two or more subways, exiting each time and re-boarding to reach work places. Getting to LAX is impossible, unless a train rider takes a bus called the LAX FlyAway.

Having the Green and Crenshaw light-rail lines connect with LAX is Metro’s No. 1 goal. But helping eastern county, Orange County and Riverside County travelers reach LAX has fallen into SCAG’s hands, a planning agency that represents six Southern California counties.

“I fly in and out of LAX and it is embarrassing,” Ikhrata said. “Every other city has a rail option into and out of the airport. So I say let’s connect commuter rail people, too, so they can use the airport.’’