If the infrastructure is key to the war effort, fair game. But like, the nether the Houthis nor the British are directly involved in the Israeli invasion of Palestine. Whereas I'm pretty sure Ukraine–having been invaded by Russia on false pretenses–has limited their targets to inside Ukraine and Russia.

Also, like, terrorism has a definition. The purpose behind Ukrainian attacks are direct military goals, while the Houthis are trying to generate pressure on the international community to drop support for Israel. The Houthis are using bottom-tier logic, but it is fundamentally different from what Ukraine is doing both in terms of actions and reasoning.

To be fair one of these is probably not transporting civilian goods if the train is close to the front lines.