The Paris Metro lines using rubber tires are almost all retrofitted (line 1, 4, 6 and 11, only line 14 was operated with rubber tired trains from the start). R

The Paris Metro lines using rubber tires are almost all retrofitted (line 1, 4, 6 and 11, only line 14 was operated with rubber tired trains from the start). Retrofitting actually has some advantages for old lines with short station distances, tight curves and steep gradients, whereas new lines can be planned with the limitations of rail technology in mind.