The strikes are the latest in a wave of industrial action to hit Germany, where high inflation and staff bottlenecks have soured wage negotiations in key parts of the transport sector, including national rail, air travel and public transport.
Train drivers began fresh strikes at 2am (0100 GMT) on Thursday, with national rail operator Deutsche Bahn warning travellers that it could run only a small fraction of its usual services.
That walkout, set to last 35 hours, marks the beginning of a wave of rail strikes planned by GDL as it pushes for reduced working hours at full pay.
Deutsche Bahn has accused the union of refusing to compromise.
Airports in Frankfurt, Germany's busiest, and Hamburg cancelled their departing flights on Thursday due to striking security workers.
The Verdi union is organising that strike as well as another by Lufthansa's ground staff on Thursday and Friday.
