Thank both for those replies; from the Network Rail link it looks like Swansea to Llanelli is only shut between the 26th and 30th of Jan. (Mon-Fri), with some trains to the west resuming on the Saturday (31st Jan). Presumably therefore the track renewal work is expected to extend into Swansea station for that first week, but the viaduct refurbishment will lake longer than that? Guessing this will mean TfW don't have enough stock to divert many daytime services via the Swansea District Line once they are able to run Carmarthen-Swansea services, but no such excuse for the first five days yet very few diversions?
I note also this from near the start of the NR press release: "carries out essential upgrades to the railway" (bold added by me here). Is it too much to hope that the track layout will be improved (eg. a double junction and double track for the last little bit of the line from the west into Swansea, or perhaps some passive provision for OHLE over the Landore viaduct at a later date) rather than just a like-for-like renewal?