At one time (let's say about 25 years ago), there was a through Exmouth - Crewe service, via Cardiff, Swansea and the Heart of Wales line. Must have been at a stage when Regional Railways had a 'Wales and West' area. The through service was there to rotate 153s between the different parts of this operation.
In a similar time period, I noticed when I was waiting at Cheltenham Spa for an early morning train to London, that there was a short-lived through early morning Cheltenham - Shrewsbury service, via Cardiff, Swansea and the Heart of Wales line. It was operated by a single-carriage
DMU▸ with Heart of Wales livery.
I did idly wonder to myself whether anyone at Cheltenham wanting to get to Shrewsbury but lacking time for the scenic journey might have mistakenly joined it instead of using the normal much faster route via Birmingham. Perhaps the guard would have turfed them off at Gloucester (the first stop) on seeing that their ticket was not valid via Swansea. At least, I doubt that a ticket for Cheltenham - Shrewsbury would have been valid for that particular through train that ran Cheltenham - Shrewsbury, even in
BR▸ days when there were none of the 'xxx operator only' restrictions.