They might be able to get it "operational" but I doubt it'll be fixed and back to it's normal standard of operation.
IIRC Russia has a huge number of specialist troops purely to run and maintain their military rail network, and they do seem to be far better trained and actually equipped unlike the rest of the armed forces, but there is a lot of difference between being able to get a rail bridge back into operation and able to take what might be much lighter loads at slower speeds (and possibly requiring checks after every train), and fixing it to take the sort of loads and frequency of loads that it was designed for.
Meaning they might get it up and able to help keep supplies running, but probably at a much reduced level and unable to make up for the loss of the road bridge, so the the choice of what exactly you send over in each load becomes far more complicated and important.