The construction of the rail network in Spain was supported by public aid. A part of them were “subvenciones adicionales”, which came from the exemption of the payment of customs duties on the imports of railway equipment. The exact amount of these grants is only known after 1870 when they were rather small. In this paper, we quantified them for the period 1857-1886 in the company MZA, by then the largest in the country. Moreover, we estimate the costs of the construction of the railway lines of that company until 1869 and how much did each of the sources of funding, public and private contributed. The main conclusion is that in MZA the State’s grants were higher than the shareholders’ investment.