sentences:  By 1936 both the Burlington's "Denver Zephyr" and the Union Pacific Railroad's "City of Denver" were locked in head-to-head competition, each with new equipment and a 16-hour schedule.The Rock Island's offering was the "Rocky Mountain Limited", operating traditional heavyweight cars on a 27-hour schedule.Although mired in bankruptcy, the Rock Island introduced six new streamlined trainsets, known as "Rockets", in 1936.