The in-train repeaters in IC and ICE trains do only support the 900 and 1800 MHz band with few refurbished cars also covering the 2100 MHz band. That and given the strong attenuation caused by the metal frame of the cars as well as the metallized windows, you should not expect reliable coverage through LTE800 inside a rolling train.
We also have a separate thread about in-train cellular service in Germany: http://www.prepaidgsm.net/forum/showthread.php?t=6938

http://www.prepaidgsm.net/forum/showthread.php?t=6938

One more option that nobody mentioned before. MTX Connect data SIM card. It has about 30 countries in a coverage zone, and it's a 3-in-1 SIM card with nano-.
It seems to be more expensive than others: 0,1 EURO per 1 MB or 10 EURO per 1 day unlimited data, but it's free to get and easy, you just need to fill the form at their website.
So you should compare carriers and decide what's better for you.