The numbers are to identify tracks in the shed… If you look close, you will see "“M1"” and an “M2"”, those two tracks are Marty’s Mainlines… M1 is the outside Mainline, and M2 is the inside mainline…
If you notice, the odd numbers are on the outside mainline (except for track 3), and the even nunbers are on the inside mainline… Track number 3 is between M1 & M2.