A boy in a red shirt sitting on top of a railroad crossing bar.
Nobody is sitting
A boy is sitting as contrasted to the second sentence where it says nobody is sitting.
exists e1 e2 s1 x1 x2 x3 x4 x5 x6.(a_topic(x1) & in(x1,x2) & on(e1,x4) & of(x4,x5) & n_bar(x5) & Actor(e2,x5) & v_cross(e2) & of(x5,x6) & n_railroad(x6) & n_top(x4) & Actor(e1,x3) & v_sit(e1) & (x2 = x3) & n_shirt(x2) & a_red(s1) & Theme(s1,x2) & n_boy(x1))
-exists e1 x1.(Actor(e1,x1) & v_sit(e1) & n_person(x1))