Number 2 diesel fuel is a special grade of diesel used in high-speed engines that bear constant loads under fairly constant speeds. One common application of Number 2 diesel is in railroad locomotives.
Diesel fuel comes in three different grades, with seven different subgrades. Number 2 diesel is any diesel fuel that has a distillation temperature of 500 degrees Fahrenheit, in addition to meeting the specifications established in ASTM D-975, a standard published by the American Society for Testing and Materials. This is in contrast to Number 1 diesel fuel, which also meets the ASTM D-975 standard, but distills at 550 degrees Fahrenheit. Number 2 diesel comes in three subgrades: high sulfur, low sulfur and ultra-low sulfur. Vehicles that ride on highways use low and ultra-low sulfur diesels.