One of the buses is marked "Linea Express" while the other is marked "Metropolitan Transit System". 
The buses are marked.
The phrase "buses are marked." is the past tense of "buses is marked ".
exists e1 p1 x1 x2 x3 x4 x5 x6 x7 x8.(ne_org_system(x6) & (x6 = x8) & ne_org_transit(x8) & (x6 = x7) & ne_org_metropolitan(x7) & n_other(x5) & ne_geo_express(x3) & (x3 = x4) & ne_geo_linea(x4) & n_bus(x2) & exists e2.(Recipient(e2,x6) & Theme(e2,x5) & v_mark(e2)) & while(e1,p1) & Recipient(e1,x3) & Theme(e1,x1) & v_mark(e1) & of(x1,x2) & n_thing(x1) & card_eq_1(x1))
exists e1 x1.(n_bus(x1) & Theme(e1,x1) & v_mark(e1))