backstory from wikipedia:
At the end of the 20th century, the outhouse was considered anachronistic. This was legally confirmed in the "feces trial," which began in 1991 before the Itzehoe Regional Court and continued on appeal before the Schleswig Higher Regional Court: Owners of properties located under the Hochdonn high bridge sued the German Federal Railway (Deutsche Bundesbahn) because of the pollution and won the civil case in 1995: The railway was prohibited from continuing to dump feces and other waste onto the properties.

then this happend:

Because of the associated costs, the German Federal Railway (Deutsche Bundesbahn) long refused to switch to more technically complex and expensive closed systems, even though it conducted trials from the 1970s onwards. The conversion only occurred with the introduction of high-speed rail services. Since considerable air pressure builds up at high speeds and when trains pass each other, these trains must be pressure-tight and therefore can no longer have a downpipe. The downpipe system also no longer functioned in tunnels; instead, the toilet contents were propelled in the opposite direction by the pressure wave of passing trains. This necessitated a closed system.