Using trains for a days trips from Prague is quite a good option. They’re quite comfortable and still cheap compared to other EU countries. If you’re planning a train journey or using trains more frequently consider buying a Z CARD for around 300 CZK (valid for 1 year). This card gives you quite a high discount (around 40 percent) for all train journeys. More info here. Note that the price of a return journey is almost same as for a single ticket. There are a few types of trains. Rychlik (R), Specny (Sp) and Expres (Ex) trains are faster ones. Be careful and never take Osobni (Os) for longer journeys as they stop at every station and this makes them extremely slow. Other trains are SuperCity (SC), EuroCity (EC) and InterCity (IC). If you’re taking one of those ones you need to pay a supplement. Trains departure and arrival information can be found on the following websites:
official czech railways website (in English)
idos - all transport timetables (only in Czech)

Tickets for local train services within the Czech Republic are purchased on the spot.

Train stations in Prague:
Praha Hlavni nadrazi (Main train station)- located in the city centre; near Wenceslas square. All trains on the east-west route stop here (trains from Vienna and Munich)
Praha Holesovice - located north of a city centre; just three stops by a metro from Hlavni nadrazi. This is a terminal station for trains coming from north (Dresden, Berlin)
Praha Smichov - located at Smichov, about 15 minutes by metro from the city centre. Somewhat smaller station for trains departing from west (the train departing for Karlstejn)
Praha Masarykovo nadrazi - located in the city centre, near Namesti Republiky and Florenc bus terminal. (trans to and from Kolin, Kutna Hora, Hradec Kralove)

London to Prague by train
You can travel from London to Prague by train, via Brussels and either Cologne, Berlin or Frankfurt.  Take an afternoon Eurostar to Brussels and a connecting train to Cologne, then the excellent 'Kopernikus' sleeper train to Prague.  Or an early-evening Eurostar from London to Brussels, the overnight sleeper to Berlin, then a scenic EuroCity journey from Berlin to Prague with lunch in the restaurant car.  Or leave London in the morning, change at Brussels and Frankfurt for the Frankfurt-Prague sleeper, giving a convenient morning arrival in Prague with the full day ahead of you. 
For more info visit www.seat61.com