Wow, what a bridge!
Built in the 1860s, opened on the 10th October 1867 and restored many times (most recently here in 2022), Barmouth Bridge is a Grade II listed single track wooden railway viaduct across the estuary near Barmouth. It is 820m in length and lies within a Site of Special Scientific Interest (hence this cache is not actually on the bridge itself).
I'm not going to give to much away for this hide. This is a crafty cache of stealth, along a very busy, muggle-riddled public footpath. I will be interested to hear how you managed to look inconspicuous at this location.