If I recall correctly, the big determination on whether this buyout would happen was negotiations withe the UAW and CAW. Now here's my question - now that EMD is part of another company, why are these two unions still representing the workforce? It's obvious that, as part of GM, EMD's workforce would be grouped into C/UAW population, but now they're not a subsidiary of a car company, and they definitely don't produce cars. And there's got to be a union out there for workers who produce large industrial equipment, which would be a better representation of what the EMD employees do than a union for auto workers.
It requires an extensive process to change a union, including an NLRB vote by the affected employees, etc. Not something done overnight.
More to the point, many if not most, of the larger unions have expanded into crafts totally unrelated to their orgins. Specifically the UAW website says:
"UAW-represented workplaces range from multinational corporations, small manufacturers and state and local governments to colleges and universities, hospitals and private non-profit organizations."
See: http://www.uaw.org/about/uawmembership.html
for more info.