The regulations of the Eurostar train from Brussels to London state that you cannot take forbidden or dangerous objects on board. What is forbidden or dangerous seems to be different on either end of the tunnel.
I reckoned that my two small 100g gas cartridges, to be used for cooking on a camping trip, were forbidden nor dangerous. At the Belgian end of the tunnel, they agreed with me. That is, the alarm bells did not go off at the security check. At the UK end, my almost empty cartridges were removed from my backpack by a security officer. And the only thing she explained to me was that I could just not take them on the train. There was no explanation as to what the exact danger of my tiny little nearly empty gas cartridges was. I tried telling her that she should get in touch with their Belgian colleagues, to harmonize their policies. But she replied that she had nothing to do with what they were doing at the other end.
Allright, now just take a deep breath. Yes, it would make more sense if they had one security plan for the entire channel tunnel, but they just don’t. And you cannot do anything about it.
Well, then what are the exact dangers of taking gas cartridges on a train? I have done a quick round on the internet, and there seem to be two types of threats. Gas escaping from the cartridge could cause fire (but only if ignited), and the cartridge can explode when it gets too hot – in a fire for example. The latter is pretty serious as you can see from some videos on YouTube. But in both cases there already has to be a fire, or a spark, before the cartridges become dangerous items. There is no imminent threat to taking them on the train, but if something goes wrong they make the situation worse. So it does make some sense to ban them from a train that goes into a tunnel for half an hour.
To me, security regulations would make much more sense if they were accompanied with such explanations. These explanations would also enable security officers to make their point more clearly. And avoid discussions with people like me.