There currently is one option to travel by night train from Tarbes (France) to Paris (France), taking between approximately 12 and 13 hours: Namely the INTERCITES DE NUIT n° 3740 operated by SNCF.
The French national railway SNCF serves several night train routes within France.
In their night trains, they typically offer two types of carriages: seats and couchettes, with the latter available in first and second class.
In the couchette carriages, you travel in 4-person compartments (first class) or 6-person compartments (second class). The berths are simply designed in both classes and are about 1.9 meters long.
Yes, it's possible to reserve an entire couchette compartment as a private compartment. This can also be done for a group.
The main difference is in occupancy: four people per compartment in first class versus six in second class. Additionally, first class berths are slightly wider. First class passengers have access to showers at stations in Paris and Toulouse. Second class offers bicycle transport for 10 euros, which may interest cycling tourists.
The night trains don't have a dining car, so it's advisable to bring your own food or eat before departure. You can also pre-order salads, sandwiches, and drinks. First class passengers receive a small breakfast. All travelers get a night kit with refreshing towelette, dental hygiene product, sleep mask, earplugs, and a bottle of mineral water.