Repair of the many tunnels and high masonry viaducts on the eastern side of Semmering, appears to have necessitated this single-line working, although complete rerailing with long welded rails and resleepering and reballasting are being undertaken.
Source: wiktionary
The most welcome progress from a short-term viewpoint, however, is that reballasting of the Trent Valley section is almost completed and as a result accelerations of express passenger trains are likely in this summer's timetable; we hear that over this section alone it should be possible to cut 20min from the timings of three Anglo-Scottish day trains and that other services will be speeded up by 8-15min.
Source: wiktionary