When I was a teenager in the 80's I kept a list (written out by hand in a small notebook) of all the Conrail, D&H, Susquehanna and Guilford locomotives I saw and photographed running through my hometown. I recorded Locomotive model type and road number. But I probably only did it for a few months, because even then Conrail was just too big to keep track of!
there wasnt much point to keeping the list, because I would never be able to get them all. (although even you cant get them all, some might still enjoying keeping a list anyway, just as a record of what you have seen. there are different ways to look at it..)
For smaller shortline railroads, it is possible to "get them all"! I have made it a personal goal to see and photograph, in person, every Finger Lakes Railroad, Owego & Harford, and Lehigh Railway locomotive:
And it's doable! because they are small railroads, and they came into existence in my lifetime.
Others, such as the Livonia Avon and Lakeville, are sadly impossible, because they came into existence before I was born, and some locomotives on the LAL roster are already gone forever.
So, it's a matter of scale!
For a Class-1 railroad, IMO there is no point in keeping track of "regular" locomotives.
Although there are interesting "sub-classes" of Class-1 rosters that people seek out, the most notable being the "heritage units"..
I know of people who attempt to see and photograph every Norfolk Southern heritage unit:
http://www.nscorp.com/content/nscorp/en ... tives.html
That's doable! because there are a relatively small amount of them..
but the NS system is also massive! which makes it quite a challenge..
there are websites to track them, so you can know when one is nearby to go out and try to catch it:
(I've only caught three!
but I dont live very close to a NS route.)
So, it can be a fun part of the hobby! but within reason..