Question: What is the train route from Charles de Gaulle airport to Calais? I see that there is a train station inside the airport. Is there only one station or many? If there are many stations please let me know the station name and the train to board and the route.

I hope the train is the cheapest and best mode to travel from CDG to Calais, if there is any other please suggest.

Answer: 
As chx pointed out, you will need to clarify what "best" is. Cheaper? Faster? Most convenient? Most frequent?

I'll concentrate on the train part.

There are two train stations in CDG:

* "**Aéroport Charles de Gaulle 1**" is only served by the "**RER**" (suburban train / express metro). It is located in "Roissypôle", which is about halfway between Terminal 1 and Terminal 2, and close to Terminal 3. You can get to it using the free people mover from T1 or T2, or by foot from T3. The RER B line which serves the station goes through the centre of Paris, serving Gare du Nord. From Gare du Nord you have very frequent trains to Lille, as well as a few trains to Calais.
* "**Aéroport Charles de Gaulle 2 TGV**" is served by both the **RER** B like the other station and by **TGV** (high-speed) trains to many destinations around France. There are no direct trains to Calais that I know of, but there are quite a few trains going to Lille, and you can change there to get to Calais. The station is in the middle of Terminal 2, between halls C, D, E and F.

Note that in Calais there are two stations as well:

* "**Calais Ville**" is the historic train station in the centre of town.
* "**Calais-Frethun**" is the train station of the high-speed line just before entering the Channel Tunnel. It is quite a bit outside Calais proper, though there are trains going from one to the other.

Finally, as many of the options involve changing trains in Lille, there are two relevant stations in Lille as well:

* "**Lille Flandres**" is the historic train station, with both TGV and regional trains
* "**Lille Europe**" is the newer train station on the high-speed line, served only by TGV and some Eurostar services. The two stations are very close to each other, and there's also both the metro and a tram line (underground as well in that part) that link the two. The metro has higher frequencies. But unless it rains or you have lots of luggage, just walk!

So considering just trains, you have (at least) the following options:

* CDG 2-TGV to Lille Europe (TGV) then Lille Europe to Calais-Frethun (mostly TGV, but there's also a "TERGV", regional train using a TGV trainset). This is the fastest option, a bit over 2 hours, most of which is waiting in Lille Europe. Probably the most expensive as well, though that varies greatly.
* CDG 2-TGV to Lille Flandres (TGV) then Lille Flandres to Calais Ville (regional train, quite a bit slower, but probably quite a bit cheaper).
* CDG 1 or CDG 2-TGV to Paris Gare du Nord (RER B) then Paris Gare du Nord to Calais (direct by TGV) or to Lille then Calais (same kind of options as above). This is probably the one that will give you the most choices in terms of schedule, and it may be cheaper, but it's be longer an much less convenient.

There's quite a bit of variation in terms of travel time, frequency, and price range between the different options. Note that prices can vary dramatically from one train to the next even going through the exact same combination, so feel free to examine as many options as you can if you want to get the best price. Note that some fares are either time-sensitive or have quotas (like on planes), so the earlier you book, the more chances you have for a cheap ticket.
[Rome2Rio says that you can get a Eurolines bus from Paris CDG to Calais.](http://www.rome2rio.com/s/Paris-CDG-Airport-CDG/Calais) The journey lasts approximately 5 hours and is significantly cheaper than the train. Below is a screenshot from Rome2Rio:

[![Rome2Rio bus from CDG to Calais](https://i.stack.imgur.com/MLbFo.png)](https://i.stack.imgur.com/MLbFo.png)

[Eurolines](http://booking.eurolines.fr/site/SelectDestinationSrv#) agrees with this. Buses run to Calais Centre Ville, Calais Ferry, and Calais-Cité-Europe. Below is a sample quote for CDG to Calais Centre:

[![Eurolines from CDG to Calais](https://i.stack.imgur.com/yYYoW.png)](https://i.stack.imgur.com/yYYoW.png)