One of the best ways to study the development of railways in Wales is to look at the tithe maps which were drawn at the time the earliest railways were being built. Here you can see a picture of which railways were built when, although you also need to be aware of when the maps were drawn.
The Cardiff St Mary map shows the Taff Vale Railway which was built in the 1830s, but not the GWR which came later. Cardiff is much smaller, the central station is not there and the Taff river follows a different course
Meanwhile the Sirhowy line starts in the Sirhowy Iron Works just north of Tredegar on the Bedwellty map…
…and ends up in Newport,where there is also a tramway into the town as it was at the time.
In the north the maps drawn in the late 1840s show the railway to Holyhead, for example here in Dwygyfylchi, where land allocated to the railway is shown, but however the village of Penmaenmawr does not exist yet.
The railway is also clearly seen in Abergwyngregyn on the Aber map
Other maps may not show railways because they were drawn in the early 1840s, as opposed to the late 1840s, see for yourselves for example Bangor