Is it a "load" so that the occupancy detector can detect that the circuit is working even if a train is no "shorting" the rails together?
Think of it as a resister and that the occupancy detector can detect 1/ an "open circuit" - a fault, 2/ an "unoccupied circuit" - line clear, and 3/ a "short circuit" - a train or line occupied.
Of course, it makes more sense if it is actually a capacitor (providing impedance) so that it works with AC electric - of a frequency that does not correspond with "mains" frequencies (and their derivatives).
Just my speculation as I don't work in the rail industry.
Kev.