I will second what Dick H said about the Downeaster. If you know when the Downeaster is due at Exeter, you can well-predict when freights will be coming through. In fact, one day (at Rockingham Junction in Newfields), I reasoned that a northbound (well, railroad eastbound) would come through Newfields approximately a half hour after the southbound Downeaster left Exeter; sure enough, a half an hour later, a northbound freight showed up. Now, of course, there is not always going to be a freight train between Downeasters, but as this example shows, it is possible to accurately predict when a freight would be coming through Newton Junction.
Also, about DOBO - I would be trackside at Newton Junction around 7:50 PM (to play it safe). Over the past couple months, I've heard them passing through East Kingston (which is right next to Newton) between 8:00 pm and 8:15 pm. And that sucker flies... chasing it is almost comparable to trying to chase the Downeaster.