Seasonal Currency

Can some one explain to me what the green seasonal currency is used for?

Since the 1st week of getting the new train I’ve been collecting the green coins every week but….. wtf can you use them on?  Not to mention I need 6 more cards to upgrade the new train.
 
I’ve checked every week since and now we only have the excavation coins to use to upgrade the Pyramid and what a farce that is!

 I’m guessing that I and probably everyone else will have approx  500000 green coins which can’t be used as I pay for the premium pass and get everything on offer until I hit tier 38!

EA need to pull their proverbial finger out and do something constructive instead of putting up all these extra buildings that they want everyone to pay for of which I already have as I’ve been playing since the start 10 years ago.

Hope this receives an answer but won’t hold my breath.