Senedd Cymru has limited legislative competence in relation to railway services see Transport - what is devolved?
The Welsh Ministers have executive functions in relation to rail-transport under the Railways Act 2005 and by virtue of The Welsh Ministers (Transfer of Functions) (Railways) Order 2018 which transferred functions to the Welsh Ministers under the Railways Act 1993 and Railways Act 2005.
These functions are primarily connected with funding, the franchising of railway passenger services and the discontinuance or closure of railway services and certain railway assets. Executive function in respect of the rail network have not been transferred to the Welsh Ministers.
Subject to certain exceptions, the Welsh Ministers have power to make orders (known as 'TWA Orders') under section 1 of the Transport and Works Act 1992 in relation to the construction and operation of railways, tramways, trolley vehicles and certain other guided transport systems. TWA Orders may authorise matters ancillary to these guided transport systems, such as the compulsory acquisition of land, the creation or extinguishment of rights over land, and the charging of tolls and penalty fares. If, however, an application relates to both England and Wales (for example, a proposed new railway straddling the border) it would fall to be determined by the Secretary of State.