People are sitting on a train or subway.
There are people sitting.
If we know that people are sitting on a train or subway then we already know that there are people sitting - the second phrasing gives no new information.
exists e1 x1 x2.(on(e1,x2) & (n_train(x2) | n_subway(x2)) & Actor(e1,x1) & v_sit(e1) & n_people(x1))
exists p1 x1 e1 x2 x3.((x1 = x2) & Actor(e1,x3) & v_sit(e1) & (x2 = x3) & n_people(x2))