George
That is a TVS (transient voltage suppressor) designed to arrest voltage spikes. Connected between the Hot and Common just like a short. There are plate jumpers dividing the T-strip enabling 5 screw heads available for multiple railpower track connections each to Hot and Common rails around a given power district.
Circuit breakers/fuses protect your transformer from current surges, TVS protects fragile locomotive circuity from voltage spikes.