Arby's Roast Beef - what cut of meat?
The "Arby's we grew up on" was a Steamship Round. That is the entire "round primal" shown in diagram. However, several years ago they went to a processed version in order to make production more uniform, I believe it is likely still 'mostly' round that has been pressed and shaped.



You are unlikely to find a steamship round on your grocers shelves, you will need to go to a butcher. The 'whole round' is usually only purchased for restaurants and caterers, it is the cut you are most likely to see in a 'serving line' (think Furr's/Luby's etc.) as it is a very large piece. Commercially the round is broken down into smaller cuts, tri-tip, rump roast, eye of round, etc. for sale in the market. If you want to make it at home the closest you can probably get is an eye of round roast. You will definitely want to slow roast it in order to get it tender.