The network grew saturated during the 1950s. Outdated technology limited the number of trains, which led the RATP to stop extending lines and concentrate on modernisation. The MP 51 prototype was built, testing both rubber-tyred metro and basic automatic driving on the "voie navette". The first replacements of the older Sprague trains began with experimental articulated trains and then with mainstream rubber-tyred metro MP 55 and MP 59, some of the latter still in service (Line 11). Thanks to newer trains and better signalling, trains ran more frequently.