The 220-pound bomb was found on a building site about 1 mile north of the station in the area of Heidestrasse, the BBC reported.
The disruption is affecting north-south rail traffic but not the city's underground trains.
A police spokesman said houses near the bomb would be evacuated.
Experts are trying to decide whether to remove or detonate the bomb, the BBC said. Rust can make the diffusion of such bombs more difficult.
A bomb was detonated on site in Munich last year and, though the area was evacuated, the blast caused a fire and damage to nearby houses, the BBC said.
There are possibly thousands of unexploded bombs buried in German, the news agency said. In 2010, three people died when one unexpectedly exploded.