The Republic of Palau is an island country in the western Pacific, in free association with the United States. In one way or another, the country has been under US administration since it was established, and has never had its own separate currency - it has always used the US Dollar. However, Palau allows some foreign mints to issue coins under its authority; these are denominated in US Dollars but are only legal tender in Palau, so can in a way be considered to be in "Palau Dollars", even though such a currency does not officially exist.
The coins are usually dedicated to historical or general popular culture themes not related to the country itself. Many of them are in the internationally popular half gram (0.5g) of gold collector format.
The Golden Train is part of the large Golden Highlights series of specially shaped gold mini-coins by CIT Coin Invest, and is a "companion coin" to a silver two-ounce coin issued for the Cook Islands in a series named "The Journey".
The mint says about it:
Following our joyful outings on the high seas, in lofty heights and on wide roads, we are now travelling on rails with the fifth edition of the endeared The Journey series. How marvellous when loved ones wave goodbye on the platform and see the smoke billowing from the chimney as the metal giant slowly rumbles and rattles off into the wide world. Long before the most famous wizard travelled aboard the Hogwarts Express or Hercule Poirot pursued murderers in the luxurious Orient Express, one of mankind's most remarkable inventions sparked progress, adventure, and excitement - the steam locomotive. | 
Search for Palau: Gold Half Gram (2023) Train on eBay 