All aging structures are affected by different deterioration processes, which might cause a performance decrease, and eventually lack of safety, in the structures. In addition to this the changing infrastructural demands pose the Swedish Transport Administration to increase the load capacity of the railway system which then also includes the bridges. This might be carried out by many different methods where strengthening is one. A particular interesting bridge type to strengthen is the concrete trough bridge and in particular in shear between the transition of the beam and the slab. Methods for flexural strengthening of trough bridges have been tested and documented, but there is a lack of methods for strengthening the bottom slab in shear. This paper presents promising laboratory test results for transversal post tensioning of the bottom slab of a concrete trough bridge specimen, and measurements reveal a vast decrease in reinforcement strains.