Barbigha police station SHO Navin Kumar said the incident occurred when labourers were preparing a casting for the railway bridge, which caved in.
Local villagers rescued the injured labourers from the debris, the SHO said adding three seriously injured labourers were later rushed to Patna for treatment.
The Police have started an investigation into the matter after lodging an FIR in this connection, Kumar said.
"The contractor is absconding since the incident. We are trying to trace him," he said adding, all injured labourers belonged to the Nalanda district.