The trump card held by CP at this point may be to conduct a proxy fight to seize control of NS' board. The NS board, for some reason, does not have staggered terms like most companies have. This opening could be exploited by CP to load the NS board with directors more amenable to the CP proposal. Once the new directors are seated, there would be nothing to prevent them from naming Hunter the new CEO. And, under this scenario, as long as his ties with CP are severed before he joins NS, the STB won't have a say.
Curt