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Some supermarkets trim down Pork Loins and label them as Tenderloins. These can be much more tough than true pork tenderloins. Of course there is always the luck of the draw, some piggy's are just tougher than others...JJ
How long did you smoke it? Those little pieces can dry out pretty quickly. An alternative might be to smoke it for a brief time with really heavy smoke, then grill or pan broil it. Brining makes a huge difference, too. I do tenderloins on the grill or, in winter, on the Breville panini grill.
Nothing, I suspect that Chef Jimmy is correct. A true pork tenderloin is under 1.5 pounds and should cook rapidly, In my area (South East) they are packaged 2 per pack and look like this.