With Lionel, you have to read the description labels on the box or research the items in the catalogs or on the web site. Shop employees should be able to help in this regard as well. I think the labels on more recent production Standard O state that the item is Standard O in the item description label, but this has not always been the case. I wish Lionel had a better way of marking the boxes and making the Standard O items easier to find.
Andy
