I've ridden along many fine railway lines over the years, and the stretch of the Cambrian Coast Line between Barmouth and Machynlleth is one of the finest.
By reputation the rest of the line is pretty amazing; must get back there and see if it's true.
On its way the line passes close to both the Talyllyn and Fairbourne Railways, and further on it connects with the Ffestiniog Railway at Minffordd.
The most spectacular sight along this bit of the line, however is Barmouth Bridge which I cover in more detail on my Barmouth page.