It will vary from train operator to train operator. Banned on Eurostar, for example, and I know they’re banned on a lot of public transport operators. Many of them don’t like lithium batteries and/ or the idea of people riding them along the platform!

I think it will be very difficult to get definite answers without contacting every train operator you will use…

Thanks for the answer .

So London right know isn't in my plans, Im gonna start by doing Spain and Portugal.

Spanish high-speed trains also have X-ray checks before boarding. So if they don’t accept trotinettes (I don’t know if that’s the case), there’s no way to ‘smuggle’ it onboard.