The presence of overhead equipment for electrification is orthogonal; the real issue is loading gauge, a cross-section profile that defines the minimum car size that the most restrictive segment of a line must support. Usually, tunnels end up being the limiting factor, since these tend to be much costlier to design to accommodate larger cars. For instance, CSX carries double-stack containers on Northeast Corridor between Philadelphia and Washington under catenary, with a vertical clearance above top of rail of 5.54 m [1]. Compare with UIC G2 gauge, which only offers 4.65 m ATOR [2].

I don't know why parent is talking about triple stacking containers. That's completely infeasible; the tallest common loading gauge in America (AAR Plate H, or 6.3 m) doesn't even come close to fitting 7.8 m = 3 × 2.6 m (about the height of a container).

[1] http://www.csx.com/share/wwwcsx_mura/assets/File/Customers/S...

[2] https://en.wikipedia.org/wiki/File:Lademass_EBO.png