What do signs by the tracks for conductors/engineers with white backgrounds and black horizontal lines mean?

It could be a "Flanger sign" AJ. All railroads have them, and many lines have their own design. The flanger signs on the CP are small black hexagons. On the BNSF, they're black strips about 10" long by 3" wide mounted on the post at an angle so they are diagonal.

When the snow plow is being pushed in front of the locomotives, it is also plowing underneath where the wheel flanges will track. The flanger portion of the plow must be raised at road crossings and at switches. If the operator fails to raise it, things get torn up. Since these flanger hazards are under the snow, the operator raises the flanger when he sees these indicator signs since he cannot see the hazards himself. One line probably means one item to raise the flanger for. Two or three lines may mean there are two or three switches, or a combination of switches and crossings all in a row. The hexagons and or diagonal strips are likewise two or three on one post to convey this message.

But what ever the railroad, these flanger signs and symbols are always black to contrast with the white background of snow.

I have never seen one quite like that but I would guess they are 1/4 1/2 and 3/4 mile markers between the mileposts.

I never saw any flanger signs that werent at an angle but they could be.

The only thing that comes to mind is a distance marker.Handy when running in snow or fog so you know how far from a switch you are.Is the one with one stripe followed by a mile marker?