So, context: This is some time after the apocalyptic event, and society has had a little time to re-build. The apocalyptic event in question physically destroyed a large number of cities, and killed most of the human population. It did not poison the air, or make the surface world uninhabitable. While much of this world is now desert, most of the desert predates the apocalypse.
There is very little coal in this world; as a result, steam locomotives have historically been oil burners. At some point in the past, oil refining was discovered, and diesel-electric locomotives became the dominant form of rail transport. (bonus question: with the majority of locomotives using oil rather than coal, how much faster would diesel locomotives have overtaken steam?)
Considering that society has recovered to the point that trains in general have become not only possible, but worth the time and effort, how likely is it that this society is already able to refine oil to the extent needed to run diesel-electric locomotives?