I have two Athearn diesel locos that were a gift to me from my father several years ago. I have not done much with them but would like to something to improve their performance and to weather them. I know nothing about the era these diesels were put into service or how long they were used. I would like to know what the differences are in the different classifications of diesel locomotives such as a GP-2 or GP-50. Where can I go to learn about the different classifications of diesel locos?  I have attached two photos of what I have, any help would be appreciated.