Aspects of railroad operations are practically unlimited since almost every railroad is unique in some way, with its own distinct characteristics and methods of operation. Some railroads utilize signals to guide their trains, while others operate without such systems. Similarly, some rely on diesel locomotives for power, whereas others opt for alternative forms of propulsion or technology.

Each of the main three types of railroads offers a variety of choices for equipment, carefully tailored to suit specific needs based on the type of cargo they transport, the geographical challenges they face, and the technological advancements they adopt. In this section, we explore horns, signals, and the detailed anatomy of railroad equipment. 

I keep these floating around on google tabs so I can remember terms:

TrainWeb:

http://trainweb.org/mdamtrak199/horns.html

GoRail:

https://gorail.org/nuts-bolts/nuts-bolts-the-track-switches-that-power-rail

RailState:

https://www.railstate.com/the-ultimate-guide-to-railroad-classes-defining-class-i-class-ii-and-class-iii-railroads/

American Rails:

https://www.american-rails.com/signals.html

Philadelphia Chapter:

https://www.philaprrths.com/position-signals

Wikipedia - EMDs: 

https://en.wikipedia.org/wiki/List_of_EMD_locomotives#:~:text=8%20Four%2Daxle%20roadswitchers%20or,or%20Special%20Duty%20Locomotives%20(SD)

Wikipedia - ALCOs:

https://en.wikipedia.org/wiki/List_of_ALCO_diesel_locomotives#:~:text=List%20of%20ALCO%20diesel%20locomotives%20%C2%B7%20Boxcab,road%20switchers%20%C2%B7%20Century%20series%20%C2%B7%20Diesel%2Dhydraulic

Wikipedia Steam Locomotives:

https://en.wikipedia.org/wiki/Category:Steam_locomotives_of_the_United_States

Railfan Guides of the US:

https://railroadsignals.us/railroadbridges/index.htm