It currently consists of 5 lines, A to E, and interconnects with the Paris Metro. It is run jointly by the RATP[?] (the body that runs most public transport in Paris including the buses and the metro) and SNCF (French national railways).
Its use of standard gauge and overhead power supply means that trains can run on existing urban railway lines. Most of the lines are the result of connecting existing rail lines that branch out from the rail termini through new underground tunnels across Paris. Some lines use double decker trains to achieve greater cpacity.
After many years of planning, construction began on the network in the 1960s. Several small segments were built and old lines electrified; the opening of central station Chatelet-Les-Halles in the heart of Paris joined several previously built or adapted lines and marked the real inauguration of the RER system. The 1960s plan of lines was not realized in full until the late 1990s.
The network reuses some of the oldest railways in France: the Paris to Saint-Germain-en-Laye line, the Ligne de Sceaux[?] and the Bastille to Vincennes line.
See also Transportation in France.
wikipedia.org dumped 2003-03-17 with terodump