The pilot of a railroad locomotive is the very front of the locomotive, beneath the coupler and fastened to the pilot beam, the end sill of the locomotive frame, to which the coupler's draft gear is mounted. Historically, this part was nicknamed the cowcatcher, but this term is archaic and when applied to any locomotive post-1900 the term makes railroad men wince. That term in any case referred only to the pointed and wedged kind applied to early locomotives, when a collision with free-ranging cattle was an ever-present danger.
On a twentieth century steam locomotive, one of two types of pilot might have been fitted, depending on intended service.
On a road locomotive, a road pilot was used. The purpose of this was to deflect anything the locomotive might hit at speed and prevent it from knocking the locomotive to the side or worse, getting under the wheels. Pushing the obstacle to the side was always preferable, both for the obstacle hit and for the train itself; the ideal was for the locomotive to remain on the rails and avoid a potentially castastrophic derailment at speed. Road pilots were normally, as in the archaic cowcatcher, vee-shaped as seen from above and wedge-shaped in side view, to throw an obstacle up and sideways. They were of much restrained proportion compared to the original cowcatchers, though, and did not protrude ahead of the coupler in such a way as to make it difficult to couple to locomotives or cars.
Road pilots were generally bar pilots made out of steel bars or tubing; frequently, used boiler tubes were re-used for this purpose, cut and welded together. Later on, especially in the post-WW2 era, the sheet steel pilot gained in popularity, partly for its more modern streamlined look, and partly because an obstacle would slide more easily over sheet steel. At the same time, the drop coupler came into vogue for the front of locomotives; this dropped down on a hinge to fit inside a recess in the pilot. The aesthetics were good, but the drop coupler had a more serious purpose; the exposed coupler had been found to catch on obstacles struck by the train, not allowing them to be pushed aside but rather driving them underneath the locomotive. The smooth face provided by the drop coupler increased safety. Alternatively, some locomotives used front couplers that lifted up and swung back, or couplers that would swing aside and into the pilot, hiding between steel covers.
Yard locomotives, switchers and the like, were from 1915 until 1977 required by the Federal Railroad Administration (FRA) and its regulatory predecessors to have footboard pilots instead of road pilots. These had a step mounted across the front of the locomotive deep enough for a man to comfortably stand, with handholds mounted above, so that conductors and other railroad workers could safely ride on the locomotive without placing themselves in danger. In addition, some, but not all, freight locomotives were equipped with a footboard pilot, possibly so that they could be used as switchers if necessary.
Diesel locomotives have pilots too. Older, carbodied "cab units" had road type pilots, more pointed on passenger units and blunter on freight units. Switchers and hood units generally were fitted with footboard pilots until the 1977 outlawing of such, though on many railroads small snowplows were installed obscuring them. After the outlawing of footboards and pilot steps, flat pilots became the order of the day. Another more modern innovation is the installation of an anticlimber above the coupler. In steam days, the crew had the whole locomotive ahead of them to protect them in case of a collision; on a modern locomotive, the nose or short hood is all that's there. The anticlimber helps prevent an object collided with from sliding up, mounting the locomotive frame and colliding with the cab.
Some fact checking done with the Model Railroader Cyclopedia Volume I: Steam Locomotives