The ToasterProbably the most well-known railfan nickname is the "toaster", a name now applied to most if not all GE locomotives. I believe it was first applied to the Amtrak AEM-7 and P42DC locomotives due to their toaster-like shape, and the fact that GE also makes household appliances like toasters.
|Amtrak 58, Philadelphia, 2005|
|I think it's browned enough|
I've also heard "techno-toaster" for the P42DC or for a Dash-9. Speaking of Dash-9, I believe that's another railfan term.
The GMD SD40-2F is commonly known as a "red barn" because it is a cowl unit and wears the Action Red of CP Rail.
|CP 9021 in Winnipeg, MB|
EMD's SD70MAC, SD80MAC and SD90MAC locomotives are called "Big Macs" for obvious reasons. The "M" is for a safety cab and the "AC" means they use AC traction motors rather than DC.
|CP 9109 at La Salle, MB|
CP 9109 is/was an SD9043MAC.
Pretty much every EMD "E" or "F" locomotive (think E8, F7, etc.) is called a covered wagon. These are called that because they are cowl units, where the shell sits on the frame.
|Covered wagon E9A CN 102|
I've heard the SD40-2W units called "thundercows" but I don't know why.
|"Thundercow" CN 5279 on the CN Family Day train in September 2014|
This was a new one to me. I can't remember if I saw it on Instagram or Facebook, but it refers to CSX cab rebuilds like this one from Flickr:
It does look a bit like Spongebob SquarePants.
Blue DevilsThere are a set of blue locomotives, IC 2455-2466, that were leased to Conrail and Illinois Central. This explains the blue! They are sublettered for IC under the locomotive number but have the big CN noodle. Railfans call them "blue devils" (why devils? I don't know), not to be confused with "blueberries" (the dark blue BC Rail units) and "blue bells" (blue GTW units).
|"Blue Devil" IC 2460|
Individual locomotives have nicknames too. I won't list them in this post but the most famous VIA Rail locomotive nickname was...
You can read some more nicknames here. Weigh in with your own nicknames in the comments!