Caption: Ontario Northland ore hopper 6613 is seen on a CN unit ore train heading through "Mile 30" near Milton in July 1974. This car was part of a group of 61 NSC-built 3712cuft cylindrical covered hoppers built in 1973-1976 for service hauling pelletized ore in unit trains from Bruce Lake, Ontario. CN had 201 cars of the same design they also pooled with ONR for this service. Note the wheels attached to roof hatches that contacted an overhead rail and allowed the hatches to be opened and closed automatically before and after loading.

There were also 35 earlier (slightly shorter) cars of this design numbered in the ONT 6500-series, built in 1967 for ore train service from Temagami with similar CN cars.

Another ore train photo: http://www.railpictures.ca/?attachment_id=18066.