Your Model Rolling Stock "Visible"
Model Railroad current-sensing occupancy detectors work on the premise
that currents flows between the rails. Prototype railroad occupancy
detectors work the same way. However, the prototype wheelset creates
a dead short between the rails when a train occupies a block.
Obviously, we cannot short the rails because model locomotives pick up
power from the rails so the rails must remain isolated from one another.
Locomotives and lighted rolling stock inherently permits current to flow
between the rails via the motor or bulbs. However, unlighted
rolling stock does not allow any current to flow between the two rails
and is designed with some type of mechanism to insulate the rails
from each other; either the wheels are made of plastic, the axle is
plastic or an insulated bushing is installed where one or both wheels
attach to the axle.
Plastic wheelsets do not provide a viable starting point for making a
detectable wheelset and; therefore, should be changed out with a metal
wheelset. Now the problem of bridging the insulation mechanism on
the metal wheelset still exists. As stated earlier, a straight
jump of the insulating mechanism is not possible as this will create a
dead short. Therefore, the next best option is to install a small
SMT-style (chip) resistor across the insulating barrier, albeit a plastic axle
or bushing. The resistor provides an adequate electrical
insulating barrier but still allows a minute amount of electrical
current to pass between the rails that can be detected by the occupancy
circuit. To learn how to add resistors to wheelsets yourself or
where to find R-T-R resistor-equipped wheelsets, view the file below: