What is an 'instance' related to metro/train track

[Thomas_bk]

Some of the game scenerios, has as a goal, to build a number of 'instances' of train or metro track.
What is an 'instance' equal to?
Is it the distance between nodes or something like that....

 

[RenengadeDragon]

Xbox series x/PS4
* I have not seen a definition.
* When playing the new scenario in Map Pack 3, I was watching how my progress was going on moving the required number of riders. I had more than enough ridership but fell short of incidences of track placed. I placed a long Metro tunnel under the water to get the number placed completed. When the scenario was complete I deleted the unused tunnel.