... But realistically, however one plays it, four stations with a combined ridership of 131k are not going to be a very good argument for slowing down service to Neath (817k) and Swansea (2.159m) ...
I agree, if you consider the chicken came first. But it you think the egg came first ....
I would suggest that the ridership at a station is a combination of the population and (potential) travel habits of its catchment,
and the provision of reliable services to and from the right place at the right time and at a decent price. I will take your work for Baglan having a catchment issue. Llanshamlet should not have that issue from my observations; Skewen and Briton Ferry, I don't know. Should we consider Pyle here too?
From Llanshamlet ... I suspect no service will be frequent enough over the short distance to beat the bus into Swansea. Into Cardiff, you have trains at 06:45, 09:17 and 11:17 ... with three TfW unit trains in addition to the
GWR▸ London expresses whistling through in between. Stops at any one of about 07:15, 07:45, or 08:45 might just fill the gap ... return trains arriving at 16:07,18:00, 19:06 and 19:57 may not be too far off already.
Take a look at how passenger number have jumped at Manea, Parson Street, Melksham, Dronfield, Pinhoe and Copplestone (just to give a few examples) and see what lessons can be learned in forecasting Llanshamlet. Which, I note, is a long way from our "bring back the buffet car" title. Pehaps we should hatch an egg and see if we have a cygnet for a separate thread?