Here's a few:
1. Serving New Hampshire industry (riped off of a Clarendon and Pittsford boxcar's slogan)
2. We can handle it (just as long as UP doesn't find out ;D)
3. Snakes through the white river (no, there aren't snakes on the trains)
4. Bringing home the bacon to New hampsire (you mentioned canada-bound trains)
5. Direct route to Canada (see above)
6. Western railroad service (the guy with the cowboy hat on 8569 made me think of that)
Some roads never actually had slogans. IIRC, the BN, SP, Cheesie, and Conrail never had official slogans.
No one thought of "wobbly rail stinks" when it was bad, did they?