Appreciable savings in fuel costs can be achieved by using residual fuels in railroad diesel engines. Experience with residual fuels in large-bore, low-speed engines has been generally reported as satisfactory except for high-cylinder-liner and piston-ring wear. Extensive tests conducted by California Research Corp. on residual fuels in small, high-speed laboratory diesel engines showed that under high-load conditions, residual fuels gave thermal efficiencies and engine deposits comparable to distillate fuels. At low power outputs, however, combustion was poor, causing excessive deposits to build up on injectors and exhaust valves. This was due largely to poor spray atomization of the high-viscosity residual fuels at the marginal injection conditions existing at low loads. Use of a two-fuel system which supplied distillate fuel at low loads proved to be a practical way of utilizing residual fuel. Improving atomization by using much higher injection pressures gave satisfactory combustion of light residual fuel at light loads. In long-term tests under conditions simulating railroad freight service, deposits were obtained comparable to distillate fuel operation. Wear, however, was appreciably higher than encountered with distillate fuel. To investigate the high wear obtained with residual fuels, wear tests were carried out in single-cylinder laboratory engines equipped with radioactive piston rings. Data on the effect of operating conditions on wear showed that increasing jacket temperature and engine load reduced wear. In residual fuels, sulfur was found to have the same absolute effect on wear as in distillate fuels. However, because wear was much higher with residual fuels, the relative effect of sulfur was substantially less than with distillate fuels. Removal of abrasive contaminants in residual fuel by filtration or centrifuging significantly reduced wear. Viscosity by itself was found to have little or no effect on wear in these laboratory tests. Limited field test data in diesel locomotives confirm laboratory engine data regarding combustion performance, deposition tendencies, and high wear of residual fuels.