Cross section of the San Francisco-Oakland Bay Bridge, looking eastward. It shows that at least as early as 1947, highway engineers contemplated eliminating the trains (top drawing) and replacing them with more automobile lanes (bottom drawing, showing options of either 10 lanes or 12 lanes for automobiles). This drawing was included in the report "Preliminary Studies for an Additional Bridge Across San Francisco Bay", California Dept. of Public Works, 3/31/47. In contrast to the highway engineers' and GM's desires, the report concludes "it is essential that the electric railways be retained on the present bridge."
My measurements of the original of this drawing, measuring the deck width to the accuracy possible using a fine ruler, shows that the trains only took 19.88 % of the bridge =5.51 cm/(12.95 cm * 2 decks). Common practice is to round to the nearest whole feet, so this value is either 20% =24 feet/60 feet /2 decks or 19.17% =23 feet/60 feet/2 decks. Since 19.88 is much closer to 20 than to 19.17, the conclusion is that the trains took only 20% of the bridge.