I had a go at trying to create a timetable with inspiration from Rhydgaled but factoring in 'stub services'.
Standard M-F timetable:
Shrewsbury to Llandrindod/Swansea, Llandovery to Swansea
Swansea to Llandovery/Shrewsbury, Llandrindod to Shrewsbury
Advantages:
Early morning service from Llandrindod to Shrewsbury runs slightly later, for a more convenient arrival for school children, college students and other commuters.
Time between early morning arrival at Llanelli and connection time to the London Paddington train reduced from 24 minutes to 14 minutes. Arrival into Swansea at 8:14 still allowing a walk to the city centre before 9.
Reworked timetable with an extra 4 ‘stub’ services between Shrewsbury and Llandrindod at 9:45, 13:50, 18:10 and 22:39, maintaining service for commuters and creating an option for an evening out in Shrewsbury with a return service. This gives an ‘average’ of one train per two hours between Shrewsbury and Llandrindod.
At the South of the line there are four extra ‘stub’ services between Swansea and Llandeilo, of which three continue to Llandovery, giving an ‘average’ of one train every two hours between Swansea and Llandeilo. The first ‘through’ service is slightly later, allowing better commuting opportunities northbound to Llandeilo and Llandovery.
This timetable uses 4 units self-contained all day, but services could be extended to other areas if needed i.e. Crewe.
Not all passing loops used for each service (i.e. no trains timetabled to pass at Llandovery at all) which allows slightly more operational flexibility for severe delays.
Through trains are later, meaning there’s an option to return later after a day out to stations Cynghordy – Builth Road than currently exists.
All long distance trains are still scheduled to pass at either Llanwrtyd or Llandrindod, allowing for a crew change.
Limitations:
Some long dwell times at loops and some long turnaround times at Llandovery, Shrewsbury and Swansea. Ideally there would be another loop at either Pantyffynnon or Pontarddulais – But with the current infrastructure, this is not necessarily a bad thing as it can help to minimise delays snowballing on the line.
Not a clockface timetable. As the loops are not evenly spaced, this makes it virtually impossible without having to skip stations. Therefore there are some larger gaps greater than two hours, but I’ve tried to keep these at quieter parts of the day. The first through services are later, meaning a later first service between Llandrindod and Llandovery, but these stations are very quiet, and the service still runs at a time the railway would usually refer to as “peak” anyway.
Some services run quite close to existing services on the Marches or SWML. I’ve tried to leave gaps of 7 minutes where possible, further speeding the services up by omitting Gowerton or Church Stretton if appropriate. Passengers for these stations can change at either Llanelli or Craven Arms respectively. Some trains may have to be retimed slightly.
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So in short, one or two regular users may be put off by the retimes of the central section of the line, but I think growth at the North and South of the line will outweigh this and open the line for more possibilities in the future. The times that have been bolded are times that the trains are scheduled to pass between Craven Arms and Llanelli.
I ran into lots of problems, and have corrected (I think) all the mistakes. The biggest problem were trains scheduled to pass not at loops (and I had to retime a lot) and I made the mistake sending a train out from Swansea before it had even arrived

- I considered starting it from Llanelli but in the end I timetabled it so it would leave Swansea later, and passing on the double track near Llangennech/Bynea instead of Llandeilo. It was a challenge to make and there may be a couple of mistakes - Though I have gone through and checked.