There were a number of EL freight trains that regularly ran via the Cutoff in the 1970s. Some symbol freights even alternated. EL employees on another list say that even trains with traffic for Ford at Mahwah sometimes ran on the Lackawanna side (the Cutoff). Going back to the Lackawanna era, the Boonton Branch was always the freight route west from northern New Jersey. Building what was called the Boonton Low-Grade Line was one of the first things the DL&W did after acquiring the old Morris & Essex in 1868. The line was completed and in service by 1870. I don't like to use Wikipedia as a reference but this time I'll make an exception.
Link And don't forget, in the EL era there was plenty of commuter train interference on the Erie side too.
There were problems with both routes. The Lackawanna side had grades west of the Cutoff and some trains required helpers. The Erie side required an extra brakeman west of Suffern because of NY State full crew laws. The Lackawanna side had more online industry. The Erie side west of Port Jervis had many curves and a grade (Gulf Summit) that also required some trains get helpers. Towards the end EL did not want to maintain two routes and didn't have enough money to do so anyway.
There was a fairly well-documented effort by a private group to buy the Cutoff from Conrail in the late 1970s or early 1980s in order to run an intermodal service. The intermodal terminal would have been located near Port Morris. It's a long story but I think the bottom line is, it failed because the group could not get enough financing. Investors thought the chances of the service being profitable enough to pay back the investment was too questionable.