6-9. In intermodal transportation, loaded truck trailers are shipped between railroad terminals on special flatbed carts. Figure

6.38 shows the location of the main railroad terminals in the United States and the existing railroad tracks. The objective is

to decide which tracks should be "revitalized" to handle the intermodal traffic. In particular, the Los Angeles (LA) terminal

must be linked directly to Chicago (CH) to accommodate expected heavy traffic. Other than that, all the remaining

terminals can be linked, directly or indirectly, such that the total length (in miles) of the selected tracks is minimized.

Determine the segments of the railroad tracks that must be included in the revitalization program.