These are plastic ties from Sicut Co. You can get some detailled information from [
www.sicut.co.uk]
The Ffestiniog Railway has installed plastic ties instead of wooden ties for some years due to the decreasing quality and durability of wooden ties available today. This was told to me when we visited this fantastic railway in July.
I‘m employee at the Hamburg Subway Co./Germany and for our outside tracks we also changed from installing wooden ties to concrete ties in general and plastic ties at special locations like bridges. The lifecycle of these ties should be twice as long as that of todays creosote treated wooden ties.
We also started installing plastic ties at the heritage railway I‘m volunteering for (Bruchhausen-Vilsen - Asendorf) for the same reason. You can use the same (historic) fastening system (screws or spikes) as with wood, so the appearance of the track doesn’t change much.
Of course plastic ties are much more expensive, but installing new ties is a pretty hard job and you really won‘t like to do this too often at the same place again....