Michael Sol told me in an e-mail that back in the 1960s, The Milwaukee Road had a hot freight that ran from Chicago to Seattle that beat the competition by several hours *AND* they did that on bad and deteriorating track with slow orders appearing.
Imagine what they could've done if they'd had good track !
Well, the Milwaukee had a train (XL Special) that had a SCHEDULE faster than the competition, but that's about all you can say. The schedule was 55.5 hours from Chicago to Seattle which meant the train had to average 39 MPH the entire way. Slow orders notwithstanding, it approaches nearly an impossibility considering the numerous permanent speed restrictions (45 MPH or less all the way from Haugan, MT to Malden, WA, and about half of that isn't mountainous territory even). Reality: The train often ran late. And, to make the schedule, the train had to be overpowered as to not require helpers at the train's four major grades. And of course, only some of the traffic going to the Pacific Northwest was going to Seattle or Tacoma. Everywhere else - like Great Falls, Spokane, Bellingham, Portland, Vancouver, BC, Yakima, Pasco-Richland-Kennewick, Salem, Albany, Eugene, Longview, etc...., the Milwaukee route was more circuitous or the Milwaukee didn't go there at all (mostly the latter).
But it didn't take long for the competition to catch up. CB&Q/GN matched the running time of the XL Special 4 years later at 55.5 hours. The difference was that while the XL Special was restricted to 3,000 tons, CB&Q/GN train 97 was restricted to 5,500 tons as far as Spokane (where Portland traffic was set out). GN could do that with the same or less power than the Milwaukee needed as its route across Montana had a maximum grade of 1% instead of the Milwaukee's 2%. A fast running time is no big deal when you are expending more resources to move the same amount of freight than the competition.
In the Spring of 1971, BN created the Pacific Zip, scheduled from Chicago to Seattle in only 50 hours, and regularly made the trek in 46. To counter this much faster service, the Milwaukee threw in the towel (at least in published schedules), putting the XL Special on a Chicago-Seattle timing of 63 hours 45 minutes, and it was downhill from there.
Same story for the Olympian Hiawatha. Initially, it was scheduled to match the Chicago-Seattle schedule of the Empire Builder of 45 hours, which it did match for several years. But by the time the Olympian Hiawatha was discontinued in 1961, its running time had improved only 5 minutes to 44 hours, 55 minutes, while the Empire Builder was making the Chicago-Seattle trip in 42 hours, 50 minutes, over 2 hours faster.
Between Chicago and St. Paul, the Milwaukee did have the primary USPS mail contracts for the Pacific Northwest, but at St. Paul, they were interchanged to Great Northern for the trek to Seattle and Portland due to speed and reliability.
The bottom line is this: A claim to the "fastest" is only valid if it can be maintained. A one-off is only anecdotal. Without doubt, Great Northern has proven to be the "fastest" in passenger, mail, and freight consistently since the end of WWII.
And instead of stating "Imagine what they could have done if they'd had good track," the salient question is: Why didn't they have good track? And the answer is: You're less likely to when you're the high-cost railroad to the Pacific Northwest.