I took another look at the Bahn page to which I sent you a link, and it doesn't really seem to apply to your case. I did find a page for call-ahead "Gepäckträger-Service", but they only showed 8 main stations where you could do that. Frankfurt Flughafen was not one of them. I certainly wouldn't expect it at Steinach.
I would expect getting on the train with luggage to be a bigger challenge in some cases then what to do with the luggage on the train. Of course, most of the express train (ICE/IC/EC have platform level loading, and some of the regional trains now are double-decker cars with platform level loading, but I don't think you can count on that. The old style couches, where you have to go up several steep steeps to enter the car are still around. (However, the last time I was on the Steinach-Rothenburg route, the were using single-decker couches with platform level loading.)
And, as I said in my previous posting, I have rarely been on a train so crowded that I couldn't put my ETBD convertible carryon bag on the seat next to me. At 64 I guess I'm fortunate not to have a problem stowing the bag in the overhead rack, but realistically, I've almost never needed to. If a train is really crowded, I think you will find someone nice enough to help you with it.