Valid question, though I can say that the following trains probably seem to be getting a big no from SWR for extn. Cant blame them as it seems to be a tit for tat approach for all the blocks that SR might be creating for SWR trains. Only people at the highest level will know
12683 SR proposed for extn till SMET . Seems like SWR rejected the same
'16319 SR has proposed for extn . SWR sited constraints
12257-...
more... Earlier I do hear that SR was willing to take over the maintenance of this train ( heard this many years ago, but not sure of its authenticity) but not cleared by SWR.