WHAT ARE THE PARIS RER TRAINS?
They are a network of 5 express train lines RER (pronounce: EHR-uh-EHR, ('Réseau Express Régional' or Regional Express Railway) complementing RATP's (the city's transport authority) very efficient Metro system. These lines cross from the outskirts of Ile-de-France from one side to the other, north to south and east to west, through underground tunnels in central Paris.
They run from 5.30 am until 30mins after midnight, 0.30am.
They are used mainly by locals and tend to be ignored by tourists even though they are a good and often quicker way of getting between main travel stations.
RER lines also connect Paris with its two busiest airports Roissy-Charles de Gaulle (CDG) and Orly (ORY) plus intercity train stations including Gare du Nord, Gare de Lyon, and Gare d'Austerlitz, so the chances are it might very well be the first mode of transport you use when you arrive in Paris. :
The entrances to the RER are identified by posts carrying a sign RER in white on a blue background. The same ticket works here as on the metro.
To access the platforms you must pass through the ticket machines. On the RER, your ticket is checked on the way in and on the way out. Each line is identified by it colour, number and directions.
They are used mainly by locals and tend to be ignored by tourists even though they are a good and often quicker way of getting between main travel stations.
RER lines also connect Paris with its two busiest airports Roissy-Charles de Gaulle (CDG) and Orly (ORY) plus intercity train stations including Gare du Nord, Gare de Lyon, and Gare d'Austerlitz, so the chances are it might very well be the first mode of transport you use when you arrive in Paris. :
The entrances to the RER are identified by posts carrying a sign RER in white on a blue background. The same ticket works here as on the metro.
To access the platforms you must pass through the ticket machines. On the RER, your ticket is checked on the way in and on the way out. Each line is identified by it colour, number and directions.
The RER's five lines are colour-coded:

Red (A1, A2, A3, A4, A5)
Main stations/stops on this line in central Paris are Chatelet-les-Halles and Gare de Lyon. It runs west to La Défense A1,3,or 5 ( then onto suburbs) and east to Marne la Vallée - the stop for Disneyland Paris- line A4.
Main stations/stops on this line in central Paris are Chatelet-les-Halles and Gare de Lyon. It runs west to La Défense A1,3,or 5 ( then onto suburbs) and east to Marne la Vallée - the stop for Disneyland Paris- line A4.

Blue (B2, B3, B4, B5)
Runs north (B3) to Charles de Gaulle airport and south (B4) to Orly Airport (via the Orlyval train from 'Antony' station) then on to Saint Remy sur Chevreuse. Main stations/stops on this line in central Paris are Gare du Nord, Chatelet-les-Halles and
St.Miche-Notre Damel.
Runs north (B3) to Charles de Gaulle airport and south (B4) to Orly Airport (via the Orlyval train from 'Antony' station) then on to Saint Remy sur Chevreuse. Main stations/stops on this line in central Paris are Gare du Nord, Chatelet-les-Halles and
St.Miche-Notre Damel.

Yellow (C1, C2, C3, C4, C5, C6, C7, C8)
Main tourist stations/stops in central Paris on this line are St. Michel-Notre Dame, Musee d'Orsay, Invalides and Eiffel Tower. This line also for the chateau at Versailles C5 and for Gare d'Austerlitz.
Main tourist stations/stops in central Paris on this line are St. Michel-Notre Dame, Musee d'Orsay, Invalides and Eiffel Tower. This line also for the chateau at Versailles C5 and for Gare d'Austerlitz.

Green (D1, D2, D3, D4)
Main stations/stops on this line in central Paris are Gare du Nord, Chatelet-les-Halles and Gare de Lyon. This line serves north and south suburbs of the city.
Main stations/stops on this line in central Paris are Gare du Nord, Chatelet-les-Halles and Gare de Lyon. This line serves north and south suburbs of the city.

Purple (E1, E2, E4)
Main stations/stops on this line in central Paris are Haussmann-Saint-Lazare which is connected to Gare Saint-Lazare (train station) via walkways and Magenta which is connected to Gare du Nord (train station) via walkway station links. This line serves suburbs to the east of the city.
Main stations/stops on this line in central Paris are Haussmann-Saint-Lazare which is connected to Gare Saint-Lazare (train station) via walkways and Magenta which is connected to Gare du Nord (train station) via walkway station links. This line serves suburbs to the east of the city.
Summary
The letter designates the line, the number designates the branch of the line, as lines can divide into branches at one or both ends. As is the case of line B to Charles de Gaulle airport. That is why it is important you board the B3 train for this journey! Check the number relevant to you destination before boarding a train.
The letter designates the line, the number designates the branch of the line, as lines can divide into branches at one or both ends. As is the case of line B to Charles de Gaulle airport. That is why it is important you board the B3 train for this journey! Check the number relevant to you destination before boarding a train.