42101: It was a bad joke
But would the Harbour Bridge really cope with the weight of a 38 class (serious)
Even the 96 locos dead weighed more than 8,200 tons. The approach grades in the tunnel are 3.3% (1 in 30), and 1 in 42 (2.4%) in the "daylight" between Argyle Stairs and the southern portal of the arch. The arch itself has 1 in 40 grades from both portals and the climb from Milsons Point station to the northern portal is 1 in 38 to 1 in 42.
No steam locomotive has ever been authorised to work through the tunnels, to my knowledge, because of the smoke and heavy gradients, not even on the "Bring Back 3801" tour in 1966, where a 46-class electric hauled the special train across the Bridge to St Leonards where the electric handed over to a pair of non-streamlined 38-class locos.
I was unaware of the 59-class + 85-class trip.
According to a paper by Margaret Simpson, Curator of the Powerhouse Museum, presented at the Engineering Heritage Seminar at the Powerhouse Museum in Sydney on 22 September 2005, there was a proposal in 1930 for Locomotive No.1 to run over the Bridge for its opening, but this proposal did not proceed. [See paper entitled "Locomotive No.1 - The First Steam Locomotive in New South Wales"].
Even electric and diesel loco-hauled trains have been kept to a minimum due to the skeletal nature of Town Hall and Wynyard stations.