If it was "as fast as the Mallard" it would be in the 'Guinness Book of Records' as 'joint record holder', but
it isn't, so can't have been.
(image removed)
The Mallard speed record is a long standing joke in the steam industry. Just because the Mallard holds the "official" record, does not mean it was not broken.
It's important to note, that after the Mallard, no other railroad made an attempt at the record, so no other accounts can be held as official. However, I have heard stories of the Union Pacific FEF's breaking the record across Eastern Wyoming and Nebraska, and the Southern Pacific GS-4's hitting speeds exceeding 110 MPH on the daylight service. And due to the Hiawatha's routine service speed of 100+ MPH, it would not surprise me if the more modern engines on the Hiawatha did not at some point break the record.
The only reason the Mallard still holds the record is because it was an official attempt, and is based on an average after making runs in both directions. The record itself was set, going downhill, with 7 mostly empty cars. The GS-4's and the FEF's have been reported to exceed the record, going uphill, with 10+ cars.