Modern trains do not use "fuel oil" in the traditional sense of heavy, viscous bunker oil used by large ships; instead, they primarily run on diesel fuel or electricity. Most non-electrified locomotives today are "diesel-electrics," which use a large diesel internal combustion engine to turn an alternator, creating electricity that powers the traction motors on the wheels. These engines use ultra-low sulfur diesel (ULSD), which is a highly refined version of the fuel used in trucks, though it is often dyed red for tax purposes (as it's for off-road use). Historically, some late-era steam locomotives were converted from coal to burn "heavy fuel oil" because it was easier to store and more efficient, but these are now mostly found on heritage or tourist lines. In 2026, the industry is rapidly shifting toward even cleaner options, including renewable diesel (HVO), hydrogen fuel cells, and large-scale battery systems. While "fuel oil" is a term sometimes used colloquially, the modern rail industry is strictly focused on high-efficiency liquid fuels or direct electrification via overhead wires to meet modern environmental standards.