As Hartsfield-Jackson International Airport looks to add a Concourse G and eventually even more concourses, it is launching a $307 million project to shorten wait times for the Plane Train people-mover and add capacity.

The Plane Train extends from the domestic terminal to the international terminal and all of the concourses in between, including T, A, B, C, D, E and F.

But during busy travel periods, the people-mover stations can get crowded and some travelers end up jostling to get on the train.

As the world's busiest airport grows even more, there will be even greater demand on the Plane Train system.

The people-mover train inside the terminal has been chugging along for about 36 years, since its inception in 1980. Hartsfield-Jackson has invested millions in an upgrade of the people-mover to reduce downtime from outages.

Now, as part of the airport's $6 billion master plan, it is adding a "turn-back" area for trains that will help increase the capacity of the tracks.

As it stands now, a train arriving at the domestic baggage claim station at the end of the line must wait for any train there to depart, before pulling into the station. That's because there's no separate area for trains to move to when another train is arriving.

But Hartsfield-Jackson plans to extend the tracks to allow space for trains to turn back, allowing a shorter interval between trains. The interval between trains is currently 108 seconds, and the extension will shave 13 seconds off, making it 95 seconds between trains when the project is complete, according to the airport.

Unfortunately, the extension won't add any new and exciting destinations for passengers taking the people-mover train.

But the addition of the turn-back area will allow 11 more cars to be added to the people-mover system -- allowing it to carry more passengers as the airport expands.

The track extension will require building a 600-foot tunnel, along with evacuation stairways. First would come a design phase. Construction would start in spring 2018 and is expected to be complete in 2019.

The tunnel will go underneath the existing MARTA station and Sky Train station. (The SkyTrain is Hartsfield-Jackson's other people-mover, which as the name implies goes above ground to the Georgia International Convention Center and the airport's rental car center.)

"This isn’t like digging a tunnel underneath normal existing structures," said Hartsfield-Jackson spokesman Reese McCranie. "We’re talking about lots of complex moving parts," with other construction projects to occur simultaneously, including construction of an InterContinental hotel next to the terminal.

"There’s a lot of complexity around this project, and this is just one part of it," McCranie said.

As it prepares to tackle the engineering feat, the airport is in search of construction firms specializing in transportation tunnels, inviting tunneling contractors to an informational meeting Sept. 20.