When we watch a movie in a theatre, we are given the ability to choose a seat of our choice, as long as there are vacant seats. We aren’t given this feature or facility in train books. Why is this so?
The Answer is Physics
Booking a train seat is quite different when compared to booking a theatre seat. A theatre is a static object, a hall that doesn’t move, while the train is a dynamic objectthat experiences motion, and in turn, is subject to various forces of nature. Thus, there is a real safety concern that has to be addressed in a train. It is not a question of seat availability. The ticket booking software for the Indian railways is designed to ensure that tickets will be booked in an orderly manner with respect to the load on the train. The distribution which ensures the best spreading out of the weight is what is aimed at.
To make things clear, let us take the help of an example. Assume there are train coaches numbered T1, T2 T3…T10, and each coach has 72 seats. When a ticket is booked, the software assigns a seat first in the middle coach, usuallyT5, and the middle seat, which will have a number between 30 and 40, and finally, it prioritises the lower berths. This makes every coach have a low centre of mass (which increases stability), and the entire train becomes very stable, even if the other seats remain un-booked.
The software selects seats ensuring that every coach has a uniform distribution of passengers and the seats are filled from the middle (around 36), moving outwards to the gates (around 1-2 or 71-72), from the lower to upper berth. The Railways have to make sure that there is balance and that every coach should have an equal distribution of load. This is the reason for you getting allotted a berth that is upper and near the gates when you book late. It is not because someone cancelled theirs; it is because of the way the system works.
A train moves at an average speed nearing 100km/hr on the tracks. It experiences strong forces and mechanics that act on it. If T1, T2, T3 from our example are completely filled, and T5, T6 are empty, while the remainingare partially filled, when the train at high speed takes a turn, certain coaches experience maximum centrifugal force and some experience minimum.This causes an imbalance and a high risk of derailment.More technically, whenthe brakes are activated, there will be differential braking force acting onevery coach owing to theenormous difference in weight. The train’s stability is jeopardised.
There are many forces that act on moving objects. The dynamics of such bodies is vastly different from that of stationary ones. This makes itcomplicated to deal with a moving object. Having said that, there are technical solutions to the proposed problem.We have developed double decker trains as well, which create a very high centre of mass, thus low stability, but safety is still paramount, so you will not be choosing your seats until sometime in the near future.