Can someone explain what's happening here? On train WiFi, when pinging 1.1.1.1 I get a normal response, but when pinging 1.0.0.1 (both are Cloudflare DNS servers) I get a response from the train router instead.

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Right, must have been some sort of block. I was just confused when the ping didn't just fail, but returned from a different address. Never seen that before.

The train's firewall has a policy-based route that blocks pinging 1.0.0.1?

Sometimes a firewall will substitute its own choice of DNS for the user.

I was unfortunately already off the train by the time I read this. Didn't have time to investigate further.

Maybe proxy server. Do a trace route to 1.0.0.1

Can you check DNS setting on your router, normally used via ISP routing is faster.