A case regarding the shale oil explosion on a train going from New Town, North Dakota to New Brunswick has been going on for some time, as those injured in the event are trying to seek compensation from the shippers and purchasers. However, two huge oil-related companies -- the Slawson Exploration Company and the Marathon Oil Corporation -- were more recently added to the case. This is being seen as a way to pressure "big oil" into taking some responsibility for what happened.
Part of the issue on the Lac Mégantic train is that crude oil and shale oil are not the same thing. Typical crude oil isn't as dangerous. Since shale oil was being shipped, the risk of fire or explosion increased.
However, it is alleged that the oil companies did not warn the purchasers or shippers of this increased danger. Had they done so, more steps could perhaps have been taken to prevent the disaster. This doesn't absolve the others of guilt, but simply shows that the guilt may go far beyond them.
Now, it could be argued that Marathon and Slawson did not know of the increased danger. However, they've been added on the grounds that they should have known, as they had carried out the testing of the shale oil. They had an obligation to see that increased risk level and to do what they could to get that information out to those who could use it, which they did not do. Even if this wasn't intentional, it could be an example of negligence.
The addition of these large corporations to the suit is a significant one, both in terms of compensation that can be collected and establishing fault. It's important for those who were harmed, and their families, to know what rights they have.
Source: Rochon Genova, "Lac-Mégantic Class Action -- Victims Demand "Big Oil" Take Responsibility for Explosion: Marathon Oil Corporation and Slawson Exploration Added as Respondents," accessed July 01, 2016