It's actually worse than raw crude oil, it's basically the tar dregs of the refining process of the raw crude after you take out all the light oils used for gasoline and diesel. They have to use heaters to make it flow enough to get to where it's burnt:
"Bunker C, known in marine circles as simply bunker, or bunker fuel, is a waste byproduct of the crude oil refining process. It is not crude oil, but it is not very far from it. Forty to fifty years ago, the refining of a barrel of crude oil left quite a bit of Bunker C. This was considered a waste product and was sold to railroads for locomotive fuel and to marine operators as ship fuel, for just a couple of cents a gallon; at times it sold for only a fraction of a cent per gallon.
Bunker C is a thick, tarry substance with a high asphalt content; it also contains trace minerals that were in the original crude oil, but have been concentrated as a result of the refining process. It is so thick it has to be heated in order to flow reliably; if it isn't heated, when the ambient temperature is below freezing, Bunker C solidifies to the point that you can walk on top of it and not leave footprints. It's a good fuel for open- flame boilers such as locomotive boilers and ship boilers.
Most oil-fired steam locomotives used Bunker C, although in some parts of the country they actually burned raw crude oil. As noted before, Bunker C was readily available and dirt cheap. (Steve Lee, April 18, 2000, via email to The Streamliner discussion group)"
(from here:
http://utahrails.net/up/bunker-c.php)