In the last decade, Ultra High Performance Fibre Reinforced cement-based Composites (UHPFRC) have been increasingly implemented for rehabilitation and strengthening purposes, rendering outstanding results. The ease of application, along with their superior mechanical and durability properties against other cementitious materials, constitute the drivers for their successful application. Despite this field being thoroughly explored and extensive literature already being available with respect to concrete and UHPFRC solutions, with particular focus on bridges or maritime environments, research on UHPFRC combined with steel in structures such as steel decks or railway bridges has only recently surfaced. This paper provides an example of the latter: the Buna Bridge in Croatia is a 9m non-ballasted railway steel bridge built in 1893, and repaired in 1953 albeit no longer in operation. The structure was transported in a laboratory setting for testing, envisioning a later strengthening by a prefabricated UHPFRC slab, connected to the original structure by means of steel studs. Dynamic and static test will be performed prior and after rehabilitation in order to compare the efficiency of the solution and in particular the bond between the two materials. A detailed analysis on fatigue will be developed, based on the updated Finite Element model obtained form the results of the test, helping to deliver an appropriate design for the future strengthening. The results summarize the effective capacity of the girder and estimate the extension in the residual life of the beam on the basis of prediction of fatigue accumulation under regular operational conditions.