What are the differences betwenn first and second class compartments on SNCF's Intercités de Nuit?

In the "Intercités de Nuit", the former "Lunéa" trains, the only difference between the second and the first class is related to the number of bunks per compartment. In the second class, you have six bunks, and in the first class only four. It is mainly a matter of privacy. Note that if you want more privacy, you can book a compartment for private use, by means of an extra fee.

The Man in Seat 61 has a bit more details on these trains:

1st & 2nd class couchettes... Watch the video

1st class couchettes have 4 berths per compartment, upper and lower on each side of the compartment. They feel much more spacious than 2nd class couchettes, as there are only 4 people not 6 in a slightly larger compartment. Great for families or a group of friends. If you want sole or dual occupancy of a whole compartment, see espace privatif below. Couchette numbering plan.

2nd class couchettes have 6 bunks in each compartment, upper, middle & lower on each side of the compartment. Obviously, if you have any mobility problems ask for a lower berth, but the cosy top bunks up in the roof space give you more privacy and are my personal preference. Having said that, although cars are air-conditioned with adjustable temperature control, bottom & middle bunks can be cooler than top bunks on a hot summer night. Couchette berth numbering plan. You can book a whole compartment for just 4 or 5 of you if you like, see espace privatif below.

In both classes: Men & women are mixed in couchettes as you don't normally fully undress, but women travelling alone can book a berth in a ladies-only compartment if they like. If you've children, you can ask the train staff for a child safety rail for their bunk. Compartment doors have a security lock which cannot be opened from outside even with a staff key, and staff are on duty if you need them. There are washrooms & toilets at the end of the corridor.

You can also refer to RAILEUROPE, or to the SNCF website if you understand French.

For Paris-Milano you have the option of using Thello, which is a proper night train. The trains are really Paris-Venezia, but they stop in Milano.

For these, you don't really book first or second class, but you book your compartment type. You can choose between two types of compartments - Couchette or Cabin. Couchette compartments are pretty basic; cabins are basically bunk beds.

Couchette compartments can be either 6 or 4 people to the compartment; cabins always have three beds, but you can book to only have 1 or 2 people in your compartment - otherwise you will share with other passengers of the same sex.

Nominally, everything except one-person cabin is standard class, and one-person cabin is first class, but this only matters if you have a railpass and are paying a supplement to travel on the night train.

Thello leaves Gare de Lyon in Paris at 19:59 each day and arrives in Milano Centrale at 06:00 The return journey leaves Milano Centrale at 22:55 and arrives Gare de Lyon at 09:30.

Thello fares are usually more expensive than the Intercités de Nuit, but there are some relatively cheap "Smart" and "GO" fares available if you book well in advance.

The Intercités de Nuit trains are not really comfortable, be it on 1st or 2nd class. For Paris - Milano, I would prefer the daytime train. There are three direct TGV (high-speed) trains linking Paris to Milano each day. The journey takes roughly 7 hours. Note that a 2nd class TGV ride is more comfortable than a 1st class night train ride. Moreover, if you are lucky, the supplement from 2nd to 1st class TGV is small and you will get even more comfort. The daytime ride is very nice, especially the leg between Lyon and Torino. Here you can have a look at what the daytime journey from Paris to Milano is like.