Recent evidence reveals that transportation's improvements within metropolitan areas have a clear effect on population and job decentralization processes. Yet, very little has been said on how these improvements affect the spatial organization of the economic activity in the suburbs. This paper analyses the effects of transportation's changes on employment subcenters formation. Using data from metropolitan Paris between 1968 and 2010, we first show that the spatial pattern of job decentralization is reinforcing the polycentric nature of Paris: the number of subcenters grew from 21 in 1968 to 35 in 2010 and the employment growth was very intense within them. Second, our main contribution is to show that the new rail transit clearly affects the emergence of subcenters: not only does the presence of a rail station increase the probability of a suburban municipality of becoming (part of) a subcenter by 5 to 10%, but a 10% increase in municipality proximity to a suburban station is found to increase its chance to be part of a subcenter by 3 to 5%.