Karbala has no commercial airport — the usual entry points are Baghdad (about 100 km north) and Najaf, which has the closer international airport and is the natural overland start for most pilgrims.
From Baghdad, shared taxis and minibuses run all day from the southern bus terminals; the drive takes 90 minutes to two hours depending on checkpoints. From Najaf, the road is shorter (about 80 km) and lined with rest stops; during Arbaeen this is the route walked by millions of pilgrims.
There is no passenger rail service to Karbala. Foreign visitors should expect security checks at the entry points to the city, especially around religious anniversaries. Hire cars with drivers are easy to arrange in Baghdad or Najaf and the simplest way to combine Karbala with the other Shi'a shrines.