I spent this summer in Erie, and had a chance to do some railfanning. Here are some spots I frequented that were somewhat close by.
-Lake Shore Railway Museum-
Probably my favorite spot, however might not be as enjoyable in the colder weather. Lots of action between the CSX and NS lines, and usually a fellow railfan or two to chat with. Only a ~15 minute drive from Erie. Has some cool exhibits both outside and in. You can't park and watch trains from your car however.
Google Maps Link:
https://www.google.com/maps/place/Lake+ ... 79.8384941
-Lake City, PA-
This is a small public park about 30 minutes outside of Erie. You can definitely park & watch here. There are also some places to eat nearby. This location only has the CSX mainline.
Google Maps Link:
https://www.google.com/maps/place/Lake+ ... 80.3453435
If you want to see the new GE engines, simply driving by the plant might be your best option. Test runs are not very frequent, and most of the new locomotives are made in Fort Worth, TX now.