Although not nearly as common as diesel locomotives, gas-mechanical or gas-electric locomotives have been used in the past. These include many mining/industrial switchers, most of the doodlebug railcars, rail speeders, the Galloping Goose motor cars, road-rail vehicles, GE's early gas-electric boxcabs, and a large amount of modern MOW equipment. Adding support for these would be a matter of creating a separate entity type that mostly uses the same parts as diesels, but burns a different set of liquid fuels.

These fuels would be gasoline, ethanol, natural gas, or other, lighter substances that work in spark-ignited internal combustion engines but are damaging to use in compression-ignited diesel engines. In this case, it may make sense to restrict diesels to burning diesel, biodiesel, and other, heavier fuels (but not as heavy as crude oil).

Gas locomotives may also require spark plugs or other new parts, depending on how complicated you want their construction/maintenance to be.

EDIT: With the new diesel warm-up mechanics, gasoline locomotives would have the major advantage of being able to cold start. However, they would generally be inferior in terms of power, speed, and durability. Think of them as the portable generator of locomotives.