Neither CN nor CP have ever said such a thing.
It doesn't even necessarily need to be a design requirement per se. Just look at all of the trackage around Philadelphia - almost all of it has overhead wire above it, and a lot of it runs double-stacks and autoracks every day, even though the wire installation predates those railcar designs by many dozens of years.
Oddly enough, that doesn't seem to be as much of a concern to the railways. The standards around 25kV overhead are extremely well defined. Hazardous materials are carried every single day under overhead wire in Europe by both diesel and electric locomotives.
No, the bigger concern for CN and CP is how the overhead - and more importantly, the ancillary structures likes poles and gantries - will affect their maintenance and clean-up operations. Will the cleanup of a derailment require additional downtime in order to restring the wire, in the event that it might interfere with the cranes and other equipment?
Dan