Twin disc instruments are an essential tool for evaluating third body materials in the wheel/rail interface. Twin disc testing results (from two different scale instruments) were proven to correlate exceptionally well with field performance of Top of Rail products and Gauge Face greases. Furthermore, the twin disc instrument was utilized to evaluate the friction characteristics (creep curve) and Rolling Contact Fatigue (RCF) propagation potential of the TOR materials. Common environmental contaminants (leaves, clay, iron oxides) were studied for their potential to cause low adhesion conditions in presence of water. A correlation between material hardness and resultant friction levels was determined.