Railways have an even lower marginal cost than planes.
DB would rather have a person travelling in the seat for 'free' and encourage ridership than have it empty.
Airlines used to think this way - the marginal fuel cost for a seat was something like $10 (essentially it only costs you at takeoff). But then airport improvement fees, security fees, faa upgrade fees meant that they were paying out $100s for passengers who were travelling for free.
Just hop on the next train: BahnCard 100 offers 12 months travel on all DB services. It only costs 3,990 EUR for standard class and 6,690 EUR for 1st class.
They also have 25% and 50% discount cards available. The 50% is quite a good deal, as I think I paid mine back within the first week of getting it.
http://www.bahn.com/i/view/DEU/en/prices/germany/bahncard.sh...