I have a video of a train stalled on a hill with 3 Amtrak P42DC locomotives and about 18 cars. The grade was around 2.2%. I talked to the engineer and asked him why he stalled. He told me it was too much wheel slip. He said he never even got it out of notch 4. They had to get two GP40-2’s to aid the 3 Amtrak P42DC’s just to pull the train up the hill.
The Norfolk and Western 611 pulled a train of at least 18 or more cars up one of the steepest grades( Saluda Grade) in America unassisted. It also stalled near the summit and managed to get the train rolling again from a dead stop! That is not only amazing it is almost unbelievable.
Steam is still king of the rails. Diesels may have more tractive effort and horsepower but they still can not get over the problem of wheel slip no matter how many anti wheel slip devices they have on the locomotive. They are also limited by weight with the majority of them topping out at around 210 tons. If your modern diesel locomotives weighed 325 tons like the Big boys then I would believe they could pull a 18 car up the Saluda grade unassisted. But as a former Union pacific conductor that traveled thousands of miles on the main line with many brand new locomotives, I can say 100% nothing in their inventory can come close to pulling what that J class steam locomotive ever did.