The RATP had initially planned to convert all of its subway lines to rubber tired operation. However, after converting lines 11, 1, and 4, the RATP came to realize that the process of converting the entire subway would take so long, it was not a worthwhile project to pursue. (It did decide that all future lines could have rubber tires though, so Line 14 was constructed for that type of operation.) However, it decided to convert one last line from steel wheels to rubber tires, the mostly elevated Line 6. The RATP also had to order rolling stock for this line, and that stock was the MP73. The MP73 Stock is similar to the MP59 Stock, but has a few modifications to make the car fare better when running on the elevated line. The first MP73 Stock was delivered in 1974. All MP73 Stock currently operate on Line 6.