As far as there only being 25 articles on the Northern Pacific, I find this strange, seeing how the N.P. was the second Trans-Continental railroad.
I don't think it is strange at all. I am guessing there is a direct correlation between the population of the territory a railroad ran through and the number of modelers of that railroad. NP ran through from pretty desolate territory as far a population goes. For modeling competition that same territory was also occupied by the GN, Milwaukee, C&NW, Soo, DM&IR, Union Pacific, and Southern Pacific.
Another thing is that while the NP pioneered some great locomotives the 2-10-0, 4-8-4, and 2-8-8-4. And although they didn't invent the wheel arrangement the best 4-6-6-4s ever built. They do not capture the imagination like the Pennsy S2 and T1 and ubiquitous GG1, or the Union Pacific's 9000 class and the big blows. On the Diesel front one compares the pine tree schemes to a Santa Fe War bonnet or SP Daylight and it isn't even a close run. They finally hit a home run with the Two tone Loewy Passenger scheme but that is such a small portion of their fleet.
As a real railroad the NP isn't known for much other than poor management, repeated scandals, and multiple bankruptcy (which means of course the NP of 1980 is technically not the same NP company that completed the transcon). Contrast that to its "owned" road the CB&Q which from its conception as the Aurora Branch never missed a share holder dividend even in the depression era.
The NP did not choose glamorous names of its passenger trains that only went to Seattle and Portland. No City of Los Angeles, Super Chief, or Broadway Limited serving the movie starts at the peak of Hollywood and Broadway. No Zephyrs, Cannon Balls, or Rockets. What do we get - The North Coast Limited -- yawn.... The Mainstreeter -- What is that anyway? Certainly doesn't make me want to ride it, let alone dream about it.
And finally whats up with the Monad? Certainly turned off a whole generation of modelers who fought in the Korean war. My father could not stand to look at the NP logo without reminding him of "the enemy".
Add to that the other model railroad things like the horrible rendition of an F7 by Athearn. The choice of AHM to do the Butter Knife passenger car scheme other than the Loewy. And the total lack of a NP prototype caboose until Atlas did the wide vision in what 1983?.
I believe all these contribute to the lack of interest in the over all modeling community.