There are four ways to end up in a lower berth. Three of them depend on availability or on somebody's discretion; the fourth depends only on finding the right person.
Ranked by how reliably they work:
1. Book it properly in the first place
Berth preference at booking is a preference, not a reservation. Selecting 'lower' does not hold one for you.
What does help is the option to book only if a lower berth is allotted. If the trip genuinely depends on a lower berth — an elderly parent, a knee that will not take a climb — that setting is far more dependable than a preference, because it fails loudly instead of quietly giving you an upper berth.
2. The lower berth quota, if you qualify
Senior citizens, women aged 45 and above, and pregnant women are allotted lower berths automatically where available, in sleeper and AC classes. There is nothing to apply for — it follows from the passenger details you enter.
Two things commonly go wrong here. Ages have to be entered correctly for the system to apply it. And 'where available' does the heavy lifting: if the lower berths are gone by the time you book, the quota cannot conjure one.
3. Ask the TTE once you're on board
After chart preparation the TTE knows which berths are genuinely empty, and can allot them. They are also specifically directed to give vacant lower berths to senior citizens holding a middle or upper berth.
Ask once the train is moving and the first round of checking is done. Be straightforward about why you are asking. And do not offer money — it is not a fee, it is a favour within the rules, and offering turns a reasonable request into something else.
The limitation is simple: on a full train there may be nothing vacant at all.
4. Swap with a co-passenger — the one that doesn't depend on luck
The other three all need something to be free. This one does not. It needs one person on your train who would rather be up top — and on a full coach, there is almost always at least one.
Regular travellers often prefer the upper berth: you can sleep whenever you like, nobody sits on your bed during the day, and there is no foot traffic past your head. Trading with them costs neither of you anything.
Your ticket does not change. Your PNR does not change. You simply occupy each other's berths, and TTEs generally treat a consensual same-class swap as routine.
The only difficulty is reach: asking around your bay covers about six people, after you have already boarded. Posting your berth on your train's board covers everyone travelling that day who is looking for the opposite trade.
Common questions
Who gets a lower berth automatically?
Senior citizens, women aged 45 and above, and pregnant women are allotted lower berths automatically where they are available, in sleeper and AC classes. It depends on availability at the time of booking.
Can I pay extra for a lower berth?
No. There is no fare difference between berth types within a class and no official way to pay for a lower berth. Anyone asking for money to arrange one is not acting within the rules.
What's the most reliable way to get a lower berth on a full train?
Swapping with a co-passenger. The other methods all require a berth to be vacant, which on a full train it usually is not. A swap only requires one person who would rather have the upper berth you are holding.
Railway rules change from time to time and a lot is left to the discretion of on-board staff. Treat this as a traveller's guide, not an official ruling — check with Indian Railways or IRCTC if something matters to your journey.