Yeah a lot of places try to pass off a pastrami sandwich as a reuben, but it really isn't. I consider myself a bit of a reuben connoisseur. I have a long standing rule that if I am eating lunch somewhere and they offer an actual corned beef reuben on the menu, I have to get it (unless I have already tried it from that particular establishment, in which case I will still most likely get it but I don't have to). A real reuben also needs to be on pumpernickel rye bread (that super dark almost black rye), or at least a swirl rye bread (swirl of dark and light rye). A normal light rye bread just doesn't cut it, and don't even try to use a non-rye bread (that is a capital offense).

So RE the pastrami / corned beef distinction. Most sandwich places already have thin sliced pastrami on hand for other sandwiches, so it is understandable they try to shoehorn pastrami into a reuben rather than carry corned beef for only that purpose. You will typically have better luck at a more full menu restaurant that might actually have corned beef on the menu separately. It is not just the "corning" process that is the difference (the boiling/brining process) - corned beef is not sliced thin, and is a lot more fatty. A proper corned beef reuben will not have thin slices of meat, they should be nice thick fatty slabs :)