Beware that there is no LH presence at most (all?) train stations. I was not allowed to online checkin QDU-FRA-IAH (ex Dusseldorf). Not clear why, but maybe related to a pending PlusPoints upgrade request. UA disclaimed all responsibility, and I didn't fancy sitting on the phone with LH at 10pm local time before a 6am train. Deutsche Bahn office at Dusseldorf train station was unhelpful, suggesting I go on the train and try to plead my case with the conductor.

Complete abdication of corporate responsibility here.

BTW, I ended up paying $900 to change the ticket to depart DUS airport (probably could've gotten this lower but had schedule constraints; D class ticket).