I believe the truck in question with the boards on the sidewalk is an early concrete truck. Some of the original ones had horizontal drums like this and when the concrete was delivered it was shoveled out or hoed out.

Later, the slanted drum with flights inside that we see on modern trucks was invented. When drum rotation is reversed the inside flights automatically bring the concrete to the end and unload it.

The wood on the sidewalk was probably to protect it from spilled concrete and the steel wheels on the wheelbarrows of the day.

I'm going with cement truck.

Looking closer at the truck and planks across the sidewalk, the theories by Nick and sean c of a delivery operation seem compelling, particularly considering the last plank on the left, which is tilted to provide a kind of ramp. Click to enlarge.

I'm puzzled by what may be going on there too.
What's on the back of the truck on the curb by the boards?

It looked like a tank at first with the curved sides, but it seems to have removable side panels. The top seems different, but the rectangle propped on the back of the truck looks like the right size to be a cover for the top and may match the sides but that seems odd since there does seem to still be some addition curved tank structure there. Maybe those panels are used as some kind of guide/ramp/chute (like a cement truck) and are just being stored there?

Then there is the big disk that seems to be swung out on a frame. The back cap to the tank? But if it's a tank, what is the dirt for? I can't think of why you'd carry dirt in a tank vs a truck bed with a canvas cover. You couldn't open the entire back of the tank if there was liquid in it either.

So they're transporting a solid that needs to be contained in a tank? What could that be?

Is the nearly legible sign in front of the truck legible on the full sized scan? I can see the words but not read them.

I would guess that they are delivering coal. Boards to avoid black dirt on the boardwalk same for the door behind the truck to protect the streets.

Definitely up to no good.

I'm puzzled by the arrangement of boards covering the sidewalk, going from the building on the far left to the truck that looks like it has a rear hatch open. The sidewalk appears smooth and level, so what is the purpose of the walkway?

[Shovel, wheelbarrow and scattered dirt indicate a sidewalk excavation has taken place. -tterrace]

I see 3 roadsters and 2 phaetons. The phaeton with the split windshield is a Dodge.

Having underground electrical connections for the streetcars created a much nicer street view in DC than other cities. Not an overhead power line in sight.