Prestatyn, Welshpool, Knighton and Chepstow are accessible by railway and there are several local bus services that serve some, but not all, of the smaller towns and villages along the Trail. Bus services in particular can be very infrequent and subject to timetable change at short notice.
Your principal means of researching railway information is via http://www.nationalrail.co.uk/ and for buses use Traveline http://www.traveline.info/ Traveline can also be contacted by calling 0871 200 2233.
The National Trail website has a very useful diagram showing all of the bus and rail links to the Trail http://www.nationaltrail.co.uk/offas-dyke-path/public-transport-map Note, however, that the 783 bus route shown on the diagram, serving Ludlow, Knighton and Bishop’s Castle, is no longer in operation).