Diesel engine sludge has received wide coverage in the literature for truck, bus, and passenger car applications. Aside from creating a dirty working environment, it has been associated with filter plugging and, in extreme cases, oil gelation. Sludge is also important to North American locomotive operators where zinc-free oils are used. In recent years, some operators have experienced sludging and filter plugging. Here we discuss railroad industry trends affecting the engine oil, review the literature on sludge formation, and characterize used oil and sludge from railroad applications. We then provide the results of a field test demonstrating the impact of the engine oil additive.