This is Los Angeles
Municipal Warehouse No. 1, in Los Angeles Harbor. (Wikipedia)
You reminded me of a sign there.
The white sign over the left portal is a warning that workers must not stand on top of cars going into the building. I don't remember the exact wording though, and this photo from Wikipedia doesn't have that detail. At least the left and right portals were for rail; I'm not sure if the central one was originally rail or was always a ramp and platform between the rail lines.
I could go check it out in the morning, if anyone cares...
And while looking this up, I discovered that the Port of LA was looking, back in mid-2020, to have it redeveloped. (Sigh. It's on the National Register of Historic Places, but that doesn't provide all that much protection.)