This is a difficult matter. I'm actually on the Open Data side (not the "dark force"), but let me play the devil's advocate here.
* What if the data will be used to convince people to use less trains? Busses and car sharing are now a huge competition. What if someone would take the data and represent it in a way very unfavourable for the long-distance trains? There are multiple ways to "tweak" the statistics without actually lying.
* DB is not a single entity but rather a complex network of smaller companies and departments within them. At the moment there's no Open Data directive from the top so they basically have to decide on their own. This has at least a few problems for department heads:
* responsibility - you can easily get your head off for a wrong data sharing decision;
* losing data as the asset - departments often use data they have as a leverage when negotiating with other departments and companies, it's a valuable assed;
* data ownage question - you normally don't have "pure" data, you have data coming from somewhere else which you enhance/process, so before sharing that you have to clear if you actually have the rights to. Having a few dozen "historically grown" data sources, this may be very difficult;
* have to pay to prepare the data for sharing. Like, convert gigabyte-sized Acces "databases" into something more usable, specifying interfaces and so on. This is a huge effort, and who pays for that?
* Next, there are a few political problems like "non-discrimination" EU directives. Every public stunt for DB Netz, for instance, may lead to a "discrimination" suite from some operator. So there's a risk of legal problems.
* DB gets a few billions a year from the state to maintain and improve the infrastructure (look up LuFV). What if some clever reporter will use the data and prove that DB could have invested and maintained better? And you're getting a few millions fine from the state/EBA?
The list goes on. For decision makers the risks and dangers are obvious but the benefits are not, or at least not yet. So I'm very glad about the rising "hackathon" movement here as it is a good way to show that there may be real and direct benefits. This may really help to persuade people.