People inside a subway car hold on to the rails while people outside the subway car walk away.
people are riding a cactus
People can't be inside of a subway car and riding a cactus at the same time.
(x)
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_cactus(x2) & n_people(x1))