First Generation refers to

First Generation refers to the diesels that actually replaced steam, like F units and GP7s, GP9s, RS-3s etc.

Second generation refers to the engines that came in long after steam was done, and typically replaced the older engines. eg. GP30s, SD40s, C424, C630, etc.

I guess you could say we're in the 3rd generation now, with AC4400s, C44-9Ws, SD70s, "GEVOs" etc. displacing those second generation SD40s off a lot of larger railroads.

Generation has to do with what the diesels replaced

First generation diesels replaced steam from 1940 - 1960.

Second generation diesels replaced first generation diesels, from 1960 - 1980.

Third generation diesels replaced second generation diesels, from 1980 to present.

This is the basic idea - the dates are very approximate.

Some railroads went from steam to 2nd generation

Something I have found to be interesting is that some railroads, Norfolk and Western in particular went from steam to second generation diesel skipping the first generation entirely.  It wasn't surprising since the primary comodity they hauled was coal, and so they hung on to steam long after most railroads had switched over.  It is only important in modeling if you wonder why you can't find an N & W f-7.

I think N&W had many GP9's.

I think N&W had many GP9's.

"Modelers' talk"

My understanding is this "generation" thing is modelers' terminology, not the prototype builders'.

I would call it historian's terminology

I would call it historian's terminology - calling it modeler's terminology makes it sound like something that only "foamers" would care about. I think the use of the term "generation" when discussing the evolution of diesel design has a broader and more significant audience than just modelers.

Sure, the prototype didn't go "hey, we're designing first generation diesels here!" nor later did they go "hey, we're designing second generation diesels!"

But after the fact, a historian takes a look at what happened and sorts it into developments and trends using terminology often not used by the participants themselves. That helps summarize and encapsulate what happened. Just because the players themselves didn't use those terms doesn't make them irrelevant or improper.

Historian often summarize what happened by sorting designs into evolutionary families or "generations".

Actually, the N&W DID...

...have a tiny handful of F-units, they inherited them later on from one of the roads they acquired (Wabash, perhaps?)

Anyway, here's a few pix:

http://nwry.railfan.net/roster/nw3721.html

http://nwry.railfan.net/roster/nw3671.html

SANTA FE & Probably BNSF now

Engineers and locomotive crews don't generally even know the model designation of locomotives from the factory.  They just refer to them by the road number series ie-6000 series, 5000 series, etc.