PROCESS OF THERMAL TREATMENT OF RAILS AND DEVICE THEREOF Field of the invention The present invention relates to an in-line process for the thermal treatment of rolled rails for improving the mechanical properties in at least one superficial layer of the rail head, and to a device for the thermal treatment of rails, specifically to a device for the in-line thermal treatment of rails exiting from a rolling system. State of the art Different solutions for devices and processes for the thermal treatment of rolled rails are included in the known art, the devices and processes specifically being directed to harden the head by means of the quenching operation. Many of these devices are not arranged in-line with the rolling stands. This implies the stocking of the rolled rails and a subsequent heating thereof before proceeding to the quenching thermal treatment, with significant energy consumption and low efficiency. In other systems the devices are instead arranged along the rolling line: the rolled rail is unloaded on a roller table, which is secured to the ground; it is then withdrawn by manipulators, including elaborate leverage systems, which control the handling of the rail during the thermal treatment to which the latter is subjected; and it is finally ejected on the cooling bed or plate by means of appropriate ejection mechanisms. The rails which are heated or directly coming from the rolling mill are subjected to a fast cooling either by the use of spray nozzles, which inject a cooling fluid (water, air, or water mixed with air) on the rail head, or by immersion of the same in a tank containing a cooling fluid. When spray nozzles are used, the drawback occurs of the rail warping in the direction of the length due to a temperature inhomogeneity in some segments of the rail and due to the subsequent different thermal dilatations. When the immersion tank is used instead, a greater cooling uniformity in the direction of the length is achieved, although in any case the temperature difference between the base of the hot rail and the cooled head results in a bending of the rail; the drawback is that the manipulators employed are not sufficiently rigid and resistant to counteract and contain said bending. Another drawback of such manipulators is that, during the treatment, they are always in contact with the rail in the same fulcra thus generating undesirable "cold" areas on the rail itself. Furthermore, with all of the known devices, the throughput of the entire line is extremely low. The throughput does not exceed 12-15 rails/hour for rails which are about 100 m long. Such devices are also not structurally simple and require a considerable maintenance, both elements determining an increase in production and management costs for the device. The need is therefore felt to provide an innovative device for the thermal treatment of rails exiting from a rolling system allowing to overcome the above said drawbacks. As far as the thermal treatment process is concerned, the immersion processes provide to make a continuous cooling of the rail head, which however results in a metallurgic structure which is not uniform through the entire thickness of the treated layer. Other processes instead include the introduction of alloy elements, such as silicon and aluminium, in the steel to be treated in order to obtain the desired final features; the addition of alloying elements has the disadvantage of considerably increasing production costs. The need is therefore felt to provide an innovative process for the thermal treatment of the head of the rails allowing to increase the mechanical properties through the achievement of an improved metallurgic structure without the addition of alloying elements in steel. Summary of the invention It is the primary object of the present invention to obtain a new process of in-line thermal treatment of rolled rails which ensures to obtain a fine pearlitic structure which is uniform through a whole predetermined superficial thickness of the rail head, specifically suitable for the use of the rails in very cold environments in virtue of the improved toughness. Another object of the invention is to obtain a device for the thermal treatment of rails, placed in-line with a rolling system, which is structurally simple, has a high sturdiness and requires less maintenance as compared to the existing devices. Therefore, the present invention aims to achieve the above disclosed objects by providing a process for in-line thermal treatment of a rail exiting from a rolling system which, according to claim 1, includes the following steps: - a first cooling step in air of the rail until reaching a surface temperature of the rail head of at least 720 - a second cooling step by means of a cooling fluid until reaching a surface temperature of the rail head from 50 to 150°C above the Ar3 temperature in order to avoid a phase transformation from austenite to pearlite; - a third cooling step in air having a predetermined duration whereby the surface temperature is equalized up to the temperature of a superficial layer of the rail head, said superficial layer having a depth in the range between 15 and 25 mm from the surface; - a fourth cooling step by means of a cooling fluid until reaching a surface temperature of the rail head lower than 500 °C whereby the phase transformation from austenite to pearlite occurs; wherein said pearlite has an uniform structure with fine granulometry in said superficial layer. Another aspect of the present invention provides to make a device for the in-line thermal treatment of rails exiting from a rolling system, which, according to claim 8, includes at least one mobile trolley in turn including - a longitudinal roller table including pairs of rollers adapted to receive along the rolling axis a rail exiting from said system maintaining the rolling position thereof, said roller table being adapted to rotate about a longitudinal axis which is parallel to the rolling axis to orient the rail head downwards; - and a longitudinal tank for containing a cooling fluid in which the rail head can be immersed. Advantageously, the device of the invention includes at least one roller table allowing to guide the rail perfectly along the rolling line thus maintaining the same position with which it exits from the last rolling stand, i.e. with the symmetry axis of the rail in a substantially horizontal position. The same roller table also provides for the rigid support of the rail, the handling thereof during the thermal treatment and the unloading thereof on the cooling plate. The roller table of the device according to the invention therefore performs all of these functions in a different manner as compared to the traditional roller table which instead only serves to forward the rolled rail and therefore needs to be combined to dedicated manipulator devices. A further advantage of the device of the invention is represented in that it provides two wheeled roller tables which, alternately and axially aligned to the rolling line, allow the almost concurrent thermal treatment of two rails improving the throughput of the system. In this manner, twice the throughput is achieved as compared to that achieved with the known devices, with a rolling rate in the range between 8 and 10 m/s. Advantageously, immersing the rail in the tank for its whole length ensures the homogeneity of the treatment, the thermal distortions of the rail being virtually avoided or reduced to the minimum in virtue of the rigidity of the device, and also ensures a greater flexibility in the handling of the final cooling step, which is the most important to obtain the final desired structure. The result of a fine pearlitic grain depends on the cooling rate in this last step as well as on the deformation of the material obtained in the rolling stands. Therefore, high cooling rates are preferred, which do not lead in any case to the formation of undesired bainitic and/or bainitic-sorbitic structures. The process according to the present invention advantageously provides four cooling steps, two by air and two by water with additives or by another appropriate cooling liquid. The head of the rails obtained by this process displays the following properties: - a high hardness (340-420 HB); - a high resistance to wear; - a sufficient toughness; - a high resistance to fatigue; - a preservation of the above said mechanical properties at very low operating temperatures (up to -60 °C); - a depth of the uniform fine pearlitic structure of at least 15-25 mm; - a good surface quality and a good straightness of the rail at the end of the treatment; - the absence of surface microcracks. The dependent claims disclose preferred embodiments of the invention. Brief description of the drawings Further features and advantages of the invention will become more apparent in light of the detailed description of a preferred though not exclusive embodiments of a device for the thermal treatment of rails, which is shown by way of non- limitative example with the aid of the accompanying drawings in which: Figures 1 a and 1 b depict examples of layouts for rail production systems provided with the device according to the invention; Figure 2 depicts a side view of the device according to the invention; Figure 3 depicts a diagram showing the trend of the temperature over time for some steps of the process according to the invention both on the surface and in correspondence with a predetermined superficial layer of the head of a rail; Figure 4 depicts a diagram showing the trend of the temperature over time on a logarithmic scale for the final cooling step of the process according to the invention both on the surface and in correspondence with a predetermined superficial layer of the head of a rail; further the CCT or transformation curves are represented; Figure 5 depicts a diagram showing the trend of the temperature over time in the single cooling step by immersion, provided in the i^nown processes, both on the surface and in correspondence with a predetermined superficial layer of the head of a rail; Figure 6 depicts a temperature-time diagram showing the trend of the temperature in the first three cooling stages of the process of the invention both on the surface and in correspondence with a predetermined superficial layer of the head of a rail. Detailed description of preferred embodiments of the invention Figure 1 a depicts an example of the layout of a part of the rail production system including the device for the thermal treatment of the invention. This exemplary layout includes: - a heating furnace 1 for billets; - a billet rolling system 2 to obtain rails; - two mobile trolleys 3, 4 each including a longitudinal roller table for fonwarding, supporting and handling the rails during the thermal treatment; - a cooling plate or bed 5, on which the treated rails are unloaded; - a straightening machine 6, used to obtain the tolerances of straightness required by the market; - an evacuation roiler table 7 towards the stocl