Although at a passing glance steam locomotives may look very much alike except in overall size,
the various features on different locomotives were devised specifically to meet varying needs. For example, the size and number of wheels is one of the major variations found between locomotives. In general, the larger the driving wheels (that is, the wheels that are connected to the cylinders by rods that actually "drive" the locomotive) the faster the locomotive could go; but with the larger wheels for higher speeds, the locomotive design sacrificed tractive force, or the power available to pull loads. For greater tractive force, the driving wheels had to be smaller, but then the locomotive could not attain high speeds. As a result, freight locomotives usually had smaller driving wheels than passenger locomotives.
   
    This industrial-use locomotive from Conrock exhibits another of the many options offered by locomotive builders to meet certain precise needs. Most locomotives pulled tenders, large trailing cars which carried a supply of water and fuel for the boiler. The locomotives used in industrial plants to
haul equipment, gravel, or fuel, around the property didn't need large tenders because they were
never far from water and fuel supplies. In fact, a tender just added unnecessary length to the locomotive, making maneuvers difficult in tight industrial yards. This locomotive is equipped with a "saddle tank" arched right over the boiler to carry some additional water. This configuration made this locomotive more appropriate to its use of hauling cars full of gravel a few miles from quarry to crusher. The weight of the water in the tank also helped the locomotives pull heavy loads without spinning
their wheels. After thirty years of work, this locomotive was retired in 1955 at the order of the Smog Control Board and replaced by a diesel engine.