1. Ambala:
  - It is known for its strategic location as a major transportation and logistics center in North India. Ambala is a key junction for road and rail networks.
  - The city's railway station, Ambala Cantt, is one of the busiest in the region, and it plays a crucial role in the transportation of goods and passengers.

2. Jharia:
  - Jharia is a town in the Dhanbad district of the state of Jharkhand, India.
  - It is not typically referred to as a "transport city." Jharia is known for its extensive coal mines and is often associated with the coal mining industry.

4. Digboi:
  - It is known for its historical significance as the place where oil was first discovered in India. The Digboi oil field is one of the oldest oil fields in the country.
  - While it is not commonly referred to as a "transport city," it is important in the context of the oil and petroleum industry in India.

The city known as a "transport city" is Kandla (Option 3).