Wagah is the only land border between India and Pakistan where people from both sides can legally cross. There are thousands of people who move between the two countries every day so the border is always packed when it is open. Even a small disturbance on one side can cause a huge bottleneck at this unfortunately tiny border.
There is a train line connecting India and Pakistan that passes through Wagah. The train starts at Amritsar in India and ends at Lahore in Pakistan. I had the experience (misfortune, more like) to take this train in 2005, and boy, waiting at the Wagah border for 20 hours is pure torture. The total travel time between Amritsar and Lahore is around 3 hours, but with the immigration checks on both sides (Attari in India and Wagah side of Pakistan) the total trip takes more than 24 hours.
I remember there were two immigration counters and one X-ray conveyor for a trainload of people and luggage — with no foodstalls, no water, unbelievably hot weather, people pushing each other, policemen/border guards who seemed to have no qualms about hitting people with their batons and of course who can forget the smell. This is one of the most unpleasant journeys I’ve ever undertaken in my life. So the next time you have to travel from India to Pakistan this is what you do: take a Biman flight from India (Calcutta, Mumbai or wherever) to Dhaka and then take another flight from Dhaka to Karachi. Biman flight is not very pleasant if you’re used to the flying luxuries that are MAS and Singapore Airlines, but it sure beats passing through the shithole border that is Wagah.
Wagah is famous for the flag-lowering ceremony that takes place every evening before the border is closed. Here is Michael Palin from BBC explaining the ceremony: