With all the great reviews of some great restaurants I think for a change of pace would be a mention of some small quick meal type places. My nomination is an Asian eatery which is located on the main concourse of the Frankfurt Hauptbahnhof. It's called Asian Gourmet and has a large variety of choices. The staff is all Asian of course and serve up large inexpensive and very tasty dishes. The restaurant itself is a totally glass enclosed space on the concourse where you place your order at the front, wait a few minutes and then take your meal to one of the tables to eat. And because of the all glass enclosure you have a view of the whole train station but are separated and protected by the glass walls. On my last visit as I was enjoying my meal a group of well dressed young business types sat at my table and as they discussed their order among themselves, one of them looked at my plate and asked me in perfect German what I had ordered. MY German is slow and limited but knowing the context of what he asked I simply and knowingly said, "Nummer neunzehn. Schmeckt sehr gut." And that was what he also ordered on my recommendation.
As pointed out "Asia Gourmet" is a German chain serving Chinese food found in practically all Hauptbahnhöfen, (main train stations) as far I've seen. It's good, has decent prices/portions, reliable in quality, almost always crowded with locals and tourists, even during off meal hours. I like the eatery.