Lionel made DC train sets in 3R during the General Mills / MPC era however they were the bottom end sets. A 2 Rail locomotive can be identified as not having a center pick-up and also typically is has fixed pilots and scale size wheels where the flange is significantly smaller than a 3 rail counterpart. Weaver made their trains in 2 rail and 3 rail so you do have to pay attention.
Since you used the fixed DC output, I would think the locomotive should have functioned properly unless it has DCC which requires 18V.
