1/4 Severn Bridge Junction signal box from the front 2/4 Severn Bridge Junction signal box from the back 3/4 Scaffolding on the signal box 4/4 Shrewsbury sign at the junction Work on the 117-year-old Grade II-listed Severn Bridge Junction signal box in Shrewsbury is now complete. It is the largest operational mechanical signal box in the world, with the refurbishment its biggest in more than a decade. The £250,000 project - carried out by Network Rail and MPH Construction and part funded by the Railway Heritage Trust - has allowed the entire three-storey building to be weather-proofed. As part of the refurbishment the original single-glazed windows, installed when the building first opened in 1903, have also been replaced with new double-glazed units.