Table 1. Summary of the differences between subways and railways.

	Subway	Train
	cste.

We summarize the difference of behaviour between subways and railways. The scaling of the average interstation length of the network with the number of stations reveals the different logics behind the growth of these systems. Another difference lies in the total ridership : while it depends also on the population density for subways, it only depends on the number of stations for train networks. Finally, the size of both types of networks can be expressed as a function of the wealth of the region, represented here by the GDP . However, because the interstation length is constant for subways, the size can be expressed in terms of the number of stations or the length. In the railway networks case, the cost of stations is negligible compared to the building cost of lines, and the size is expressed in terms of the total length .