https://bikepacking.com/routes/trans-cambrian-way/

There are train stations at either end of this trail. The train from Aberystwyth and Shrewsbury calls at the remote Dovey Junction station. From Shrewsbury there are trains towards south Wales which call at Knighton, so the simplest option might be to leave a vehicle at the start and return to pick it up by train. The trains are local services and generally take bikes without booking, although it’s worth double checking with the train companies, as policies do change. The journey back, including the change at Shrewsbury takes around 3 – 3.5 hours.

There are also options for taxi minibuses from Machynlleth (5 miles by road from Dovey Junction) back to Knighton, and due to the meandering nature of the trail, it’s actually only 50 miles by road between the two towns so it’s entirely possible to use two vehicles, and leave one at each end.