well, they don't make it easy to find out do they?
Looks like the french and autoslaap services are out for unless you've gone for a roof chop (lowering brings its own problems)
German autozug is more promising but you have to meet some criteria as follows:
max 196cm height for services from Dusseldorf (well placed if your on the north sea ferries or in the Netherlands)
other services can take unto 205m height.
https://www.dbautozug.de/contentblob/2285594/bild+map/img9.jpg" onclick="window.open(this.href);return false;
with my pop-top measuring in around around 198cm height I've not explored many more options as the autozug location nearest terminal to Rotterdam is 360 miles away. (i live in york so north sea ferries are a great exit point for me)
i might explore letting some air out my tyres, but think I'm getting pretty close to saying that its not working for me at the moment.
beware lowered vehicles though, as ground clearance is an issue also. I think i read a minimum clearance of 10cm which shouldn't be a problem if your loved one is unmolested.
Full details on the autozug website, you need to attempt a booking to discover some nuances of the heights relative to pick up points and destinations as the table provided (extract below) over simplifies the situation.
Maximum vehicle height including roof attachments
to / from
Maximum roof width Germany, Austria Italy France
1.35 m(most saloons, estate cars and minivans) 2,05 m 1,97 m 1,83 m 1,75 m*
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Regarding why you might want to use the train - well last time I drove to Austria I cleared Germany in a day at 90 - 100mph, in calm and comfort on cruise control up and down some pretty steep hills in a rather warm 36C. That trip was about the destination, not the journey. in the camper though - 40mph in 3rd on hills on the autobahn is not my idea of fun, and so it becomes a German based touring holiday, or a lot of days travelling relative to time spent in destination. (Netherlands and Germany make good holidays, no disrespect intended)
I hope this helps, and would be interested if anyone with a 'compliant' vehicle uses the train
cheers
D