How would a routing such as the very limited "Brownage" (Orange from Midway to Adams/Wabash then Brown from Adams/Wabash to Kimball) and v.v. be numbered? There are only a limited number of trains each weekday in each direction. By your proposed scheme would the northbound trains be numbered 5->3 (5 to 3) and southbound 3->5 (3 to 5)?
Since you brought up NYC, the earliest trains, such as the 2nd, 3rd, 6th and 9th Avenue lines and the shuttles along 34th and 42nd Sts didn't have displayed route numbers. It seemed to work well while it lasted. New York is an outlier in that with the exception of the G , 7, Franklin Shuttle, Rockaway Shuttle, and some truncated overnight routes, trains on all lines start in an outer borough, go through Manhattan and continue to an outer borough. Almost all the trunk lines have multiple routes running as either Express or Local so there has to be a way to differentiate them. Chicago doesn't have that problem (yet?).
Since you brought up NYC, the earliest trains, such as the 2nd, 3rd, 6th and 9th Avenue lines and the shuttles along 34th and 42nd Sts didn't have displayed route numbers. It seemed to work well while it lasted. New York is an outlier in that with the exception of the G , 7, Franklin Shuttle, Rockaway Shuttle, and some truncated overnight routes, trains on all lines start in an outer borough, go through Manhattan and continue to an outer borough. Almost all the trunk lines have multiple routes running as either Express or Local so there has to be a way to differentiate them. Chicago doesn't have that problem (yet?).
"Welcome aboard Orange Line run 711. Travel time to downtown is 25 minutes. Next stop will be Pulaski."