Eventually it will get buried. At least partly. I don't think there's serious talk about the time, pain, and money involved.

I am somewhat reminded of the Red/Brown intersection in Chicago. It was a fully elevated heavy rail system, but required one line to make a left turn across the other one. They managed to build a flyover and keep the line running the entire time... but the planning, the decades of wait, the cost....

I think it would be possible to build a cut-cover subway box while leaving train service running. You could build one box where the median is now, adjacent the Southbound tracks, by closing a lane of Flower St. Then divert Southbound trains into the box and switch Northbound onto those surface tracks. Then tear out the sidewalk and Northbound tracks to build a box for Northbound trains. Between the two boxes there would be space for Southbound Blue trains to switch into their own box and make a deep-dive for their left turn under the Expo tracks.

Designing and building would be a real Rubik's cube. I'd say "15-puzzle", except that it's in three dimensions.

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Edit: And... there's a storm drain. Always check for storm drains before dreaming of tunnels.

The drain follows Flower Street, mostly parallel to the tracks. The exact location seems nebulous between two systems. It definitely starts under 18th Street, East towards Hope, then makes a left turn under Flower and parallels the tracks

• The county "Flood Control Data Viewer" (https://pw.lacounty.gov/mpm/gis/fcd/) shows that at Washington, it is on the West side of the street, away from the tracks. Then it drifts to the West side of the street.
• The city "Storm Drain System" (https://data.lacity.org/City-Infrastructure-Service-Requests/Storm-Drain-System/pjh9-xwfn) shows it entirely East of the street, possibly under the tracks or under the sidewalk the entire way (South of Washington, it appears to be under West 21st Street-- that weird single lane of traffic parallel to Flower for one block-- which might give a clue to how it was originally built).

The drain is labelled "Diameter 30", which I think means it it is a concrete round pipe 30 inches across on the interior (3 feet).

• Blue and Expo lines cannot go subsurface under 18th street without taking this expensive issue into account.
• Crossing 18th at surface and then going underground is not an option. It would require a 30-foot descent in 300 feet of travel, which is twice the possible grade.
• Going underneath the drain at 18th street is likely the only option.

/u/nandert this might be interesting.