There are many routes you can take to Disneyland Paris Resort by train. The train station at Disneyland Paris is called Marne-Le-Vallee Chessy. This station is located next to the disney village and just seconds away from the security checkpoint that leads to the parks. The bus stops for the free hotel transfers are just to the side of the station so it does not take long to get to your hotel from the station.
There is a direct train to Chessy station from St Pancras in London via the Eurostar service (although there is a rumour that this service may be discontinued in the future). The only limitations of this carefree way to travel to the magic is the number of trains running per day. There is currently one service per day direct to Disneyland so you are restricted in your choice of travel times. The cost is also prohobitive to some. The service is run as an airline, you check in and even get an in-journey meal.
One downside is the time the train leaves means getting into London can be difficult. A stayover the night before in the capital can extend your trip and take away some of the stress but also adds an additional cost onto this way to travel. You can also just get the Eurostar to Paris and change there onto another train to Disneyland.
If you choose to fly to Charles de gaulle then you can take the TGV to Disneyland in about 10 mins. The TGV is the superfast train service in France, run by SNCF. You can get an assigned seat on this service. All trains are accessed via Aoeroport CDG station which is easy to find following the signs within the airport.
Other ways would be to catch the slower trains into Paris and take the Red A line train to Chessy. If you have taken the Eurostar to Paris this is also the train you will need to transfer to. From the airport to Chessy takes about an hour to an hour and a half depending on your connections. The cost of the TGV is roughly the same as the slow train but you need to plan ahead to check the train times. If you fancy a few hours in Paris then the slow trains are the option to consider.