This cache is part of the SideTracked series. These are Cache and Dashes for public transport users, which can be found at or near train stations, ex-train stations or near miniature or narrow gauge railways such as this.
The cache is located along a public footpath between the WHHR and the WHR there is no requirement to walk on the railway or cross the line other than at recognised crossings. Please look out for trains at all times when crossing the line.
The original Welsh Highland Railway was a 1 ft 11 ½ in narrow gauge railway that operated for just 15 years from 1922 to 1936. The Welsh Highland Heritage Railway (WHHR) started in 1980s and runs trains for 1 mile to Pen-y-Mount junction with the WHR.
The WHHR operates a webcam where views of the railway and Snowdonia can be seen and the current weather conditions found out.
You're looking for a a 35mm canister.
Congrats Ecobake on FTF
GZ can get a bit overgrown in the summer so you may like to bring gloves, if you have trouble finding this cache please feel free to message me for more precise directions. Please note I have edited the hint to give a more direct hint. Please only decrypt if you need it.