While Humvees are indeed heavier than this (5200-5900 pounds,
When I left Afghanistan about six months ago, Humvees were well over 10,000 pounds. Come to think of it, they weighed about the same in Iraq in 2006.
why can't they be wider?
Right-of-ways, tunnels, etc.... Some areas (I'd imagine out West, but I've never been on a train out there) might be able to support them, but then you're stuck with rolling stock that can only traverse x% of the lines. Even if you don't have anything on the outboard side of the lines, there's still generally a fixed space between tracks, which means you'd have to widen the lines.
An Ada exception is when a routine gets in trouble and says 'Beam me up, Scotty'.