sentences:  In 1987, demand for export coal necessitated the introduction of the CB class bogie hoppers on Ngakawau-Lyttelton export coal trains.These hoppers were heavier than the dedicated LC high side coal wagons previously used, and this led to a reduction in the amount that three EOs could haul through the tunnel.In 1988, NZR began experimenting with the use of diesel traction through the Otira Tunnel, with the reallocation of DX class diesel locomotives from the North Island, redeployed following the opening of the North Island Main Trunk electrification.