How many paper mills are on the SLR now? Zero. However, they still have paper 'on-line'... NEPW operates 3 crossdocks on the SLR and Safe Handling also has one (Although not sure if it is paper that they handle). In any regard, A lot of stuff trucks into NEPW and then goes out Rail beyond our borders. Just because you don't see it interchange on rail doesn't mean they are taking 'em out!
Also, the routing depends on where it is going.... if you read a few Trains magazines ago, they had a great article on the paper industry. Quad Sussex, which is one of the larger receivers of coated paper, handles inbound rail via CN.... so a lot of the time the routing goes ST-SLR-CN, or CMQ-SLR-CN.... but you won't see the CMQ-SLR bit because it is in Canada where the interchange happens.