I've successfully completed my first 7-wide locomotive!
Power is provided by two PF train motors, which are attached to the frame via Technic plates.
The first version of this design was used at my LUG's display this year, but I changed up the internals quite dramatically based on performance issues; namely, it was very slow and couldn't keep up with standard 9v trains, and kept jumping off the track at the curves. The overall looks were improved drastically as well, in my opinion.
I'm quite pleased with how this turned out, and I hope to build more locomotives in this style in the future!