What is meant by UP train and DOWN train in Indian Railway?

Question

They use the term UP and DOWN to express the direction of a particular train. How do they assign the direction?

Like Ahmedabad to Mumbai is UP and Mumbai to Ahmedabad is DOWN - Why?

Answer 1

Maybe they don't like using North and South?

Answer 2

UP/DOWN is an almost universal traffic control convention on railroads across the world. Its used where trains only travel in one direction on each track (some sections can be both ways in which case the convention does not apply). I believe its origins are the UK where all UP lines are towards London and DOWN the opposite direction. I would imagine given the British involvement in the start-up of Indian rail that this convention was used in India (perhaps to Delhi and/or Mumbai for example is always UP).