Railroad signaling works based on detection of trains on the railroad. If a train is detected, then the signal system goes into action, setting up indications that let show that the track section is occupied and warn other trains that the section is occupied. At a grade crossing where a railroad and a highway intersect, the presence of a train starts the crossing signals and lowers the gates in time to stop highway traffic so the train can pass.
In the case of real railroads, detection is easy, since the rails are naturally insulated from each other and either a locomotive or rolling stock causes an electrical bridge between the rails that is used to operate the signals.
Two rail model railroads implement signaling by detecting electric current flow. Powered locomotives and their motors and the mobile decoders draw current, even when the locomotive is not moving. Cars with internal lighting provide the necessary detection component with their lights consuming a small amount of electric current.
Unpowered rolling stock has no electric current flow and is not so easily detected. This may or may not be important to you. Your layout design can allow extra margins for undetected cars, but if you have a series of many signalling blocks and want to operate frequent trains, then making this rolling stock "detectable" becomes important.
Resistance wheel sets have a resistive component that allows a small amount of electric current to flow from one rail to another. By doing so, a box car standing on a main track will be detected and cause the signal system to give the proper signal indication, even if there is no locomotive present.
There are a variety of ways to do this. Seaching the internet for "resistive wheel set" brings up at least one company that manufactures these wheel sets. A typical price is around $2.00 per axle. This search also brings up a variety of home-solutions which use surface mount resistors or electrically conductive paint.
Obviously, equipping every axle in your whole fleet of freight cars could be an expensive proposition. The good news is that you can probably get away with one or two axles per car.