The explosion happened after drilling during construction work near Donnersbergerbruecke station caused the bomb to go off on Wednesday, police said.
The force of the blast almost tore a seriously injured worker’s leg off, German media outlet DW reports.
Doctors operated on him for four hours, reconstructing his leg. His condition was then listed as stable.
Bavarian Interior Minister Joachim Herrmann said workers accidentally hit the 250kg bomb while drilling.
No further information was available on the type of bomb involved but munitions dating back to World War II are not uncommon finds in Germany.
More than 70 years after the end of the war, about 2000 tonnes of live bombs and munitions are still discovered in the country each year, Reuters reported.
According to official estimates, some 15 per cent of the World War II bombs did not explode, and in some cases are buried six metres below ground.
In June 2019, residents of Ahlbach, southern Germany, were awoken early one morning by a sudden blast that left a crater four metres deep in a cornfield.
Police later said it was “almost certainly” caused by a World War II bomb.
It’s been 80 years since The Blitz
Police defused an unexploded World War II bomb weighing 500kg in Berlin in April 2018, and in September 2019 almost 60,000 people were evacuated in Frankfurt after a 1400-kg bomb was found.