Question
Why IRCTC won't let you pick your favourite berth ???
Answer
The reason
1/5
After...
more...

successfully logging into IRCTC website you pray your next wish is answered. Your favourite window seat. You see there are almost 500 tickets available. You thank your stars and say no way you are losing a window seat.
Finally when you book, you don't get the window seat you opted for but you still find 499 tickets remaining. So why is this?
Getting into the details
2/5
Allotting you a seat by IRCTC isn't as easy as booking a movie ticket where you are provided a seat map. Since train is a moving object safety concerns are high. The IRCTC software has an algorithm to distribute the load evenly throughout the train.
Science behind the seating
3/5
Imagine there are sleeper class coaches in a train numbered S1, S2 S3... S10, and in every coach there are 72 seats. So when someone first books a ticket, software will assign a seat in the middle coach like S5, middle seat numbered between 30-40, and preferably lower berths.
This is because Indian Railways will fill the lower berths than upper one so as to achieve low centre of gravity.
Uniform distribution
4/5
The software books seats in such a way that all coaches have uniform passenger distribution and seats are filled starting from the middle seats (36) to seats near the gates i.e 1-2 or 71-72 in order from lower berth to upper. Railways just want to ensure a proper balance that each coach should have for equal load distribution.
That is why when you book a ticket in last you are always allotted an upper berth and a seat numbered around 2-3 or 70, except you are not taking a seat of some one who has cancelled their seat.
What if the railways book tickets randomly
5/5
A train runs at an average speed of around 70km/hr. There are a lot of forces and mechanics acting on the train. If S1, S2, S3 are full and S5, S6 are empty and others are partially full, when the train takes a turn, some coaches face maximum centrifugal force and some minimum.
This creates a high chance of the derailment of train. When brakes are applied there will be different braking forces acting at each of these coaches because of the huge weight differences, creating a disruption in the train's stability.