Track joins mess with that sort of thing. They serve two purposes: 1) to insulate between sections of the aformentioned short-across-rails signalling system, 2) to allow for the thermal expansion/contraction of track due to temperature. Even without 1, you still need 2 and that will interfere with sending any signal down the track.

The problem is that detecting that serves no purpose, and you need to detect past those joins for it to serve a use.