Le Metropolitain

Not so famous about Paris is its incredible mass transit system, whose organization is so complex it can be daunting to the foreigner.

The subway system, formerly known as the Transit Métropolitain, "Metropolitan Transit"—later shortened to Métro de Paris and thus becoming the namer of so much other underground rail systems—is world-famous for the Art Nouveau architecture of many of its stations, most of them built between 1900 and 1920, as well as for being one of the busiest and densest in the world: 14 main lines and 2 auxiliary lines crisscrossing downtown Paris, with Lines 1 and 14 being totally automated (remote-controlled, in reality). The Châtelet-Les Halles station, which serves a grand total of eight lines (Subway 1, 4, 7, 11, 14, and RER A, B and D), is one of the largest and busiest subway stations of the world. The tickets for the metro are very, very tiny.

Then, you have the RER, Réseau Express Régional, "Regional Express Network", which is a network of rail underground within Paris and ground-level outside of Paris, which serves the entire region of Ile-de-France—sort of like a faster subway. It is operated jointly by RATP, Paris's transit authority, and SNCF, France's national rail company; the difference is largely irrelevant as the transition between RATP and SNCF sections is seamless, and only matters when the RATP or the SNCF are in strike. Unlike the Métro, the RER runs on normal train cars moving through standard railroads, as it was initially planned to use the existing railroads of Ile-de-France. While not as iconic as the Métro, the RER more than makes up for it for its incredible size: a total of 587 km of railroad, serving 257 stations.

In addition, there is also the Transilien network, run by SNCF, which operates 8 shorter lines (H, J, K, L, N, P, R and U) around Ile-de-France and uses the RER's fare system. (Fun (?) fact: RER C is known as Réseau Escargot Régional (Regional Snail Network) because its length and its many branches mean even small delays will result in at least one big cascaded delay, whereas RER D line is popularly known as "RER Trash" due to its high rate of accidents and disturbances). The frequent delays due to malfunctions, incidents with users and the occasionnal strike (in 2007, the biggest strike led some people to become stuck in the middle of their trip or at work since you could wait up to four hours between two trains, if they did not decided to just stop) gives the RATP (Régie Autonome des Transports Parisiens - Autonomous Office of Parisian Transports) nicknames such as Reste Assis T'es Payé ("Keep your seat, you're paid"), Rentre Avec Tes Pieds ("Go home on foot") or Râle Autant que Tu Peux ("Complain as much as you can").

One of the main characteristics of the RER/Transilien network is its named routes. Due to the lines' length and their many branches, there are many different routes and service types a given train can take, and in order to reduce confusion, the routes (called "missions") are named with a four letter mission code. There are two naming schemes: RATP's scheme, used on RER A, B and C, and SNCF's scheme used on RER D and E as well as all Transilien lines:
 * Under RATP's naming scheme, the first letter indicates the route's terminus, the second letter indicates the service type (E means omnibus, i.e. stops at all stations), the third and fourth letter are just there to make the name pronounceable, and after the code comes a number from 01 to 99, odd when westbound (line A) or northbound (line B) and viceversa, and increased every time a train of the same mission leaves the first station; when the train counter reaches 100, the counter is resetted and the last two letters are changed. DROP24, for example, means it stops at Noisy-le-Grand Mont d'Est (D), it's not omnibus (R), and it's the 12th train that has followed this mission so far. When the two last letters are ZZ (e.g. DRZZ), this means the service has been changed for unexpected reasons such as an accident or the trains being on strike; when the first letter is a W, the train is completely nonstop and headed for the maintenance depots.
 * Under SNCF's naming scheme, meanwhile, each letter has one meaning depending on the line. On RER D, for example, MIPE indicates a route that ends at Châtelet-Les Halles (M), nonstop between Gare de Lyon and Villeneuve-Saint Georges (I), through the Évry-Courcouronnes branch (P), and doesn't calls at Viry-Châtillon station (E). There is no train counter, and code VIDE (French for empty) indicates a train that goes nonstop to the maintenance depots.

The Bus system is similarly divided in two: the RATP lines, which run around downtown Paris, and the Optile lines, which provide transportation within Ile-de-France. Of note is the night bus, which passes at 30 minute intervals when the last regular busses stop passing, previously known as Noctambus back when it only served downtown Paris, now renamed Noctilien because it now reaches out to Ile-de-France.

Finally, there are also four Tramway lines run by RATP, which operate in the inner-most suburbs in a circle, as well as a recently opened line of Vogueo boats that can be paid with a regular transport ticket and will move you along the Seine river.

Also of note is the terrible snarl that is the fare system:
 * To begin with, under this system, the Ile-de-France region is divided into six concentric fare zones.
 * A normal "T" ticket, which costs 1 €, is valid for one single bus/tramway/boat trip, or for all the subway and RER trips you want as long as you leave neither the stations nor the fare zone.
 * A "T+" ticket is similar, but at a slightly higher cost and is also valid for one rail and one bus trip (in any order), or as much bus trips as you can within an hour and a half, or one single Noctilien bus trip. From there, it all degenerates into a massive clusterfuck
 * To move from one zone to another you need a round trip ticket which is only valid at two stations (except that all stations inside Paris are equivalent), and if you have to travel through many zones—for example, from south zone 5 to north zone 5 thourgh all intermediate zones—it can get really expensive, in the order of the 13 €, which is roughly 18 dollars.
 * The Mobilis pass is valid for a certain number of zones, and for slightly higher price it will let you take as much trips as you want within the specified zones; the Ticket Jeunes ("Youth Ticket") is similar, costs half as much, but only applies in week-ends and holidays and if you're 27 or younger.
 * The Navigo card is a pre-paid pass valid either for unlimited trips for a period or rechargeable with money. There are also special low-cost passes for students, disabled people or people on welfare...

The bad part? You'd better understand and know by rote the entire fare system, because if you screw up (for example, you have a ticket for zones 1-5 and you're in zone 6), you have to pay a fine of 86 €, or in US money, $127!

And the worst part? Since this fines-abuse is VERY widespread (all those people you see peeking around the doors into the station are people who skipped the turnstiles and are scouting for railroad cops) the transit lines are often patrolled with cops (railroad security units, backed up by national police and riot police in some cases) asking for tickets.

However, this complicated system is due to be simplified to a single-fare by 2016, as figuring in the platform of the Green/Communist part of the regional coalition in power.