>I do get the impression that some leeway is now granted for timekeeping in really dense fog. San Joaquins do run slightly slower and fall behind schedule systematically.

Never heard of this before (and I ran trains down there for over 20 years), nor have I ever seen "fog so thick you couldn't see 50 feet". At night the rule of thumb is to drive your car with the low beams on because all the high beams to is light up the fog to the point where it becomes a white wall. This is even more true with the 1200-1500 watts of headlight/ditchlights on locomotives, and with the FRA's insistence that they be on bright all the time . . .