A guide to the various classes of travel
There is probably little that will shape your travelling experience on the Indian Railways as the class you decide to travel by. The railways are a good example of the heterogeneity that exists in Indian society, especially the difference in income levels. I could travel from Delhi to Mumbai for as little as INR 275 in the unreserved coaches of the Firozpur Janata Express, as well as pay fifteen times that amount (INR 4,135) travelling by First AC Sleeper in the Mumbai Rajdhani Express. There is, of course, a massive qualitative difference in the level of comfort between the two.
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are as many as ten different classes of travel that the Indian Railways has to offer, but no train offers all ten classes. I follow with a very brief guide of each class, from the most expensive to the cheapest. Also, do note that the Indian Railways operates newer LHB coaches on some trains and routes, offering passengers a vastly improved travel experience. While the fare for a particular class is the same irrespective of whether the train uses these newer coaches or the older-design ICF coaches, your journey might be quite different...
First AC Sleeper (1A)
This is the highest class of travel the Indian Railways has to offer, and unsurprisingly, the most expensive as well. First AC Sleeper consists of four-berth cabins and two-berth coupes with doors that can be bolted from the inside. Not every train has a First AC Sleeper coach - this class is restricted to the more popular trains with high demand across all classes.
First AC Sleeper is spacious and comfortable - a full First AC coach carries only 18-24 passengers (depending on the specific coach being used). The next highest class, Second AC Sleeper, has 46-54 passengers to a coach - significantly more crowded! Bedding is provided, and an attendant is available on call to attend to requests of the passenger. Each berth has a reading light. Toilets in this class are far better maintained than all other classes. As First AC Sleeper costs as much as a flight for longer journeys, it is not the most popular class for long distances. However, for shorter journeys, it can be quite difficult to get reservations on short notice..

Above, L - R: The exterior of a typical First AC coach; the corridor of the First AC coach of the Bangalore Rajdhani Express; a four-berth cabin on the Karnataka Express; a nicer two-berth coupe on the Udyan Express.
First AC - approximate berth measurements (when berths have been put down for sleeping)
Berth Length Width
Lower Berth (LB) 200 cm* 89 cm
Upper Berth (UB) 190 cm 73 cm
* Includes the handrests, as they're at the same level of the lower berth when it's made up for sleeping.
Executive Class (1A)
Executive Class is the railways' version of business class in an aircraft, and is only found on Shatabdi Express trains and a few day Duronto Expresses. It is a sitting class with 2 x 2 seating and a generous amount of legroom. As Shatabdi Expresses include catering in their ticket fares, passengers travelling by this class do not need to bother about buying food on the journey. Food served in Executive Class is of better quality (and quantity) than food served to passengers of other classes in the train. Most Executive Class coaches are extremely well maintained, and short to long day journeys by this class are very comfortable. If you can afford it, it definitely makes for a pleasant travel experience.

Above, L - R: The interiors of the Executive Class coach of the Mysore - Chennai Shatabdi; another Executive Class coach on the New Delhi - Bhopal Shatabdi; the food menu for Executive Class passengers (click to enlarge); the first course of breakfast on the Bhopal Shatabdi.
Second AC Sleeper (2A)
Second AC Sleeper is the second highest sleeping class offered by the Indian Railways, and is found on most overnight trains. It offers passengers a fair amount of space, and is perfectly comfortable for short to long journeys, as long as you get berths together.
Second AC Sleeper offers less privacy than First AC Sleeper as it lacks the lockable doors of the latter. It consists of eight bays of berths that are divided into four "inner" (or transverse) berths and two "side" berths. While the former are arranged perpendicular to the direction of the train, the latter are parallel to its movement. Each set of four inner berths has a privacy curtain that can be drawn across to cut off views of the corridor, as does each set of side berths. Each berth has a reading light (though whether it will work is another story). Like in First AC Sleeper, bedding is provided.

Above, L - R: The corridor of a Second AC Sleeper coach - inner berths on the left, side berths on the right; a set of four inner berths; the side berths folded down in the sleeping position; a nice two-berth cabin on the Rajdhani Express.
Second AC - approximate berth measurements (when berths have been put down for sleeping)
Berth Length Width
Lower Berth (LB) 185 cm* 67 cm
Upper Berth (UB) 180 cm 60 cm
Side Lower Berth (SL) 181 cm# 58 cm
Side Upper Berth (SU) 178 cm# 58 cm
* includes the handrests, as they're at the same level as the lower berth when it's made up for sleeping.
# as side berths are bound by walls on both sides, there's no scope to stretch any farther than the berth's length.
First Class Non-AC (FC)
This is a rather rare class of travel - the railways are phasing it out in favour of Three-tier AC Sleeper as it is rather unremunerative. Today, First Class Non-AC is found in a few trains in South India (mostly Tamil Nadu) and a handful of trains in the north.
The arrangement in First Class Non-AC is fairly similar to that of First AC Sleeper; the coach is divided into four-berth cabins and two-berth coupes with sliding doors that can be bolted from the inside. First Class Non-AC is relatively unpopulated - there are only 26 passengers to a coach. The huge advantage of First Class Non-AC is that it doesn't insulate from the environment like the AC classes do - your interaction with the outside environment is not disrupted by a heavily-tinted glass window. However, unlike other non-AC classes, First Class Non-AC also gives you a significant amount of privacy, as if you have a whole cabin or coupe to yourself, shutting and bolting the door totally cuts off any contact with other passengers, vendors and beggars.
The disadvantages of First Class Non-AC are that (a) since it's not AC, it gets extremely hot in summer and very cold in winter (b) all First Class coaches are extremely old, and some extremely rundown, (c) bedding is not automatically provided as in the AC classes, and while it can be obtained from the attendant on payment of a nominal fee of INR 25, finding the attendant can be difficult at times, not to mention the fact that bedding is very limited, (d) since coaches aren't insulated from the outside environment, they're usually far dustier than their AC equivalents.

Above, L - R: Corridor of a First Class Non-AC coach; a four-berth cabin; a two-berth coupe; ready to sleep!
First Class Non-AC - approximate berth lengths (when berths have been put down for sleeping)
Berth Length Width
Lower Berth (LB) 200 cm* 78 cm
Upper Berth (UB) 190 cm 61 cm
* includes the handrests, as they're at the same level as the lower berth when it's made up for sleeping.
Three-tier AC Sleeper (3A)
Also known as Third AC, Three-tier AC Sleeper is one of the most popular AC classes in the Indian Railways; it is the cheapest AC class offering sleeping accommodation barring the relatively rare AC Sleeper Economy class. It makes for a fairly comfortable journey.
Three-tier AC Sleeper has a similar layout when compared with Second AC Sleeper - the coach is divided into eight bays that have six "inner" berths and two "side" berths in each bay respectively. Unlike in Second AC Sleeper where the inner bay has just two lower and two upper berths, the inner bay in Three-tier has three sets of berths; lower, middle and upper (hence the name, "three-tier"). The backrest of the lower berth rotates upward to form the middle berth, which when up prevents any of the three passengers - lower, middle, and upper - from sitting upright. There is almost no difference between side berths in Second AC Sleeper and Three-tier AC Sleeper; except that the latter do not have reading lamps. Till fairly recently, Three-tier AC Sleeper coaches used to have privacy curtains like Second AC Sleeper, but a recent Railway Board directive has seen them removed. Currently, Three-tier AC Sleeper coaches do not have privacy curtains in the aisles as seen in the images below.
Bedding is provided in Three-tier AC Sleeper.

Above, L - R: The corridor of a Three-tier AC Sleeper coach - inner berths are on the left and side berths on the right; the lower berth rotates up to form the middle berth; the side-lower berth; ready to fall asleep in the upper berth of a 3A coach on the Goa - Bangalore Link Express.
Approximate berth measurements - Three-tier AC Sleeper (when berths have been put down for sleeping)
Berth Length Width
Lower Berth (LB) 180 cm 62 cm
Middle Berth (MB) 178 cm 58 cm
Upper Berth (UB) 178 cm 58 cm
Side Lower Berth (SL) 179 cm# 56 cm
Side Upper Berth (SU) 176 cm# 56 cm
# as side berths are bound by walls on both sides, there's no scope to stretch any farther than the berth's length.
AC Sleeper Economy (3E)
AC Sleeper Economy is a rather rare class of travel found on a handful of Duronto Expresses. The same type of accommodation is found in all Garib Rath Expresses, though rather oddly, it's classified regular Three-tier AC Sleeper (3A) there.
AC Sleeper Economy lacks the privacy curtains seen in Second AC; the layout is open-plan. A major difference between Three-tier AC Sleeper and this class is that there are three side berths per bay in AC Sleeper Economy rather than two. Thus, each inner bay has two lower berths, two middle berths and two upper berths, while each side bay has a side-lower berth, a side-middle berth and a side-upper berth. This class is fine for short overnight journeys, but for longer journeys it can start to feel slightly cramped.
In Garib Rath trains, bedding is not provided automatically - it can be rented by paying an additional INR 25 - either at the time of booking tickets, or to the attendant on the train.

Above, L - R: AC Sleeper Economy coaches from the outside; the corridor of an AC Sleeper Economy coach; an inner bay with one middle berth up; the side middle berth
AC Chair Car (CC)
AC Chair Car is a good class of travel for short day journeys. A slightly more crowded class than Executive Class, it is nevertheless fairly comfortable. It is airconditioned with 3 x 2 seating. AC Chair Car is found on regular trains as well as Shatabdi and day Duronto Expresses.

Above, L - R: The exterior of an old AC Chair Car coach (also the header image for this page); the interiors of the same coach; a newer AC Chair Car coach; AC Chair Car on the Mysore - Chennai Shatabdi Express.
Sleeper Class Non-AC (SL)
Sleeper Class is the class the majority of reserved passengers travel, and the majority of the coaches in a regular overnight train are likely to be Sleeper Class coaches. It's an extremely inexpensive way to travel, and a 3,000 km journey by Sleeper Class can cost as little as INR 800. There is far more "life" in the Sleeper Class coaches when compared with the average AC coach.
Think of Sleeper as Three-tier AC Sleeper without the AC and the free bedding. Sleeper Class coaches are designed to carry 72 passengers in an arrangement of 9 bays - each with six inner berths and two side berths. There are no privacy curtains or reading lights, and as the coaches aren't insulated from the outside environment, they can get dusty, not to mention hot or cold depending on the outside temperature. Sleeper Class coaches can sometimes get slightly crowded during the day with short-distance passengers. Then again, it's so cheap...

Above, L - R: The exterior of a Sleeper Class coach; the corridor of another Sleeper Class coach; the inner bay of the same coach with one middle berth up; the side berths - the side upper berth can just about be discerned in the image.
Approximate berth measurements - Sleeper Class Non-AC (when berths have been put down for sleeping)
Berth Length Width
Lower Berth (LB) 176 cm 60 cm
Middle Berth (MB) 176 cm 60 cm
Upper Berth (UB) 176 cm 60 cm
Side Lower Berth (SL) 174 cm# 60 cm
Side Upper Berth (SU) 174 cm# 60 cm
# as side berths are bound by walls on both sides, there's no scope to stretch any farther than the berth's length.
Second Class Sitting (2S)
Found on most daytime intercity trains, Second Sitting is a very cheap way of travelling by train - the cheapest reserved class, to be precise. It consists of 3 x 3 seating, and seats do not recline. Some of the newer Second Sitting coaches have individual seats for each passenger, though the majority of the Second Sitting coaches have bus-like bench type seats, each bench seating 3 passengers. This class often gets full with short-distance passengers without proper reservations. Second Sitting is a fun class for short (<4 hour) journeys, though for long journeys, it can get rather uncomfortable...

Above, L - R: The Second Sitting coaches of the Tippu Express; the interiors of the Second Sitting coach on the Trivandrum - Kozhikkode Janshatabdi Express.
Unreserved (UR)
This is the cheapest class of travel. As the name suggests, you do not need a reservation to board this coach - you just buy a ticket and hop on. Being as cheap as it is, it is often the only way the poorest of the country can travel between cities. Unfortunately, most important Express trains have just two unreserved coaches in their formation, leading to massive overcrowding in those coaches. Unreserved coaches are designed to carry 90 passengers, but on important trains, you might well find over 300 passengers in them ...