You need a matching set for the train and controller, it's very simple to replace the quartz with a different set. Check out the information on this site:
http://www.walts-playmobil-collection.me.uk/hints_&_tips.htm . It has a chart with the different frequencies and colors of quartz modules and has some tips and such.. I think you can still get some playmobil quartz directly from playmobil UK, but you don't have to use playmobil quartz. You just need matching sets of 27 MHz AM quartz in the size 49U. In my experience transmitter and receiver quartz are interchangeable and don't seem to be different from eachother functionally other than the printing on the shell, but I could be missing some information, they just happen to work for me however I mix them. I found a pair of matching quartz on ebay for about $5 with free shipping here:
https://www.ebay.com/itm/161797279816 .. It works fine, though non-playmobil quartz doesn't come with the color coded playmobil plastic housing which is sort of unnecessary, but might help keep moisture out of the electronics (you can of course plug any 49U quartz into a playmobil quartz housing sleeve if you want a generic quartz to have the protection of the housing sleeve). The ebay listing above does not let you choose which frequency you get, they send a random pair. The picture shows a green frequency(which would have been great), but they sent me a red/brown #3 set which is a bit close to red and brown frequencies. When I ran 3 RC cars, one with red, one with brown, and one with red/brown, the red/brown controller would sometimes control all 3 cars. Best to get colors further away from each other on the frequency spectrum shown in the chart at Walts site. If you want the cheapest option, on online shopping sites just search for "27 MHz AM quartz 49U" and you will probably find something usable for cheaper than what playmobil charges, but make sure it's AM and not FM. I'm under the impression that you can use just about any 27 MHz AM controller to control playmobil trains as well, since I've used a couple of different controllers with them successfully.
-bbbb