Dragging chains, and spring loaded wires sticking up from the pavement at toll booths, are no longer necessary. Modern-day tires aren't such good electrical insulators as their older versions were. Tires are now built to dissipate static electricity and keep it from building up on the vehicle.

The Inflammables, I mean. I've never seen it used in the plural like that, although when I was a kid in the '60's they seemed to be transitioning from that to "flammable." Nowadays they just run with the Hazmat placard. It reminds me of R.E. Lee's soldiers, calling themselves "Lee's Miserables."

Would the driver be using those smaller cans to trade full for empty cans from smaller users?

I'd guess they are five-gallon capacity, and they are probably all painted red, although some are pretty beaten up.

Looks as though the rail holding them in is permanent, so maybe they were a fixture of the relatively young delivery system which was still evolving.

[See these two forum posts. -tterrace]

Note the anti-static grounding chain dangling under the rear of the trailer. This was common practice well into the 1950's. When a tank truck passed you could hear the chain jingling as its end dragged and bounced along the road.

Many times I have seen pictures of assorted trucks on Shorpy that sported very small outside mirrors or, more commonly, none at all. Considering the volatile nature of this truck's contents it must have not been a safety consideration during this era to have some kind of mirror on the passenger side.