Trivia time again!
Difficulty: Classic Rock (if you know the oldies, and you appreciate the classics, then you know this answer without looking up wiki).
Q: What came before the GP50 and after the GP40-2?
A: While the SD family enjoyed a SD45 there was not a similar model for the GP family... but there was an X. The GP40X incorporated the latest technology of the time which would eventual be used in the GP50 and SD50, and boosted an extra 500hp over its older sister the GP40-2.
Though it there were only 20 units sold, and unlike is other kin like the GP39X, SD45X, and DDA40X, it is the only experimental unit in Class I service today.
Update: Extra Credit... Difficulty: Its my job (because I love the smell of diesel in the morning)
Q: BNSF has GP40X marked on the sides of their locomotives, but what is there true rating?
A: Sometime after BN and SF merged the 40X fleet was derated to 2500hp. On the books they are now listed as GP25X.