This cache is a reincarnation of the cache previously in this location, GC3WK77, disabled during refurbishment works, archived 4 years ago, but amazingly still in place. The CO of the original cache cymrukid is credited with its placement, but is no longer active. There has since been another cache within spitting distance (GC63B42), but that is now also archived.
This should be an easy find, it's not on railway property, but is in a very busy area, so stealth will be required at most times of the day. GZ itself is wheelchair/pushchair accessible, but retrieving the cache will almost certainly require assistance.
About Cardiff Queen Street Station
Cardiff Queen Street railway station (Welsh: Caerdydd Heol y Frenhines) is a railway station serving the north and east of Central Cardiff, Wales. It is the second busiest railway station in Wales, being located near Queen Street. It is one of 20 stations in the city and two in the city centre, the other being Cardiff Central.
In 2014, a reconstruction of the station was completed in order to reduce bottlenecks, with two extra platforms being put in (a previously existing platform, opposite platform 4, and a new bay platform next to platform 2 for the line to Cardiff Bay), taking the total number of platforms to 5.
A station known as "Crockherbtown" on this site was built in 1840 by the Taff Vale Railway, whose headquarters were also located here. It was rebuilt and given its present name in 1887. Other major rebuildings took place in 1907 and by British Rail in 1973. The latter project saw the station's overall roof removed, the original Taff Vale station frontage demolished & replaced and the western side platform closed, with all through traffic concentrated on the central island.
Information and photograph from Wikipedia article, re-used under Creative Commons license.