In introducing a post on The Transport Politic on France’s new low-cost TGV service (named OuiGo in some truly horrible Franglais), Yonah Freemark writes this of France’s High Speed TGV trains in general:
France’s SNCF national rail service has, since the introduction of the TGV in 1981, held to the belief that fast trains should not be segregated to serve only higher-paying passengers. As a result, fast trains have replaced all slow-speed service on most long-distance travel throughout the country; passengers are able to take advantage of fare deals that allow them to journey between cities hundreds of miles apart at €25 or less, as long as they book in advance.
“As long as they book in advance” is the key phrase here. Note also that “fast trains have replaced all slow-speed service on most long-distance travel throughout the country”. This is largely true; while other intercity trains continue to operate unreserved service on regular lines, their frequencies are typically poor and service patterns irregular, and often changes are required where one-seat TGV rides are available.