You can call a noise "thunder" even if it's not actually thunder: if it's just a very loud and powerful noise. We talk about the thunder of hoofbeats, the thunder of a lion's roar, the thunder of gunfire, the thunder of a great tree as it falls to the ground, and the thunder of cracking ice as icebergs collapse.
We also use the word "thunder" to mean "to say something in a very loud, powerful voice" and "to move in a loud, powerful way."
You could say that a train comes thundering into the station. That means it approaches with speed, noise, and power, in a way that reminds you of thunder. Or you could say that a herd of elephants thundered across the grass. That means they run in a loud, stomping, crashing, powerful way.
One more thing! Hundreds of years ago, a playwright named John Dennis figured out how to make the sound of thunder on the stage. It was really good—so good that another playwright stole his idea. When Dennis heard his own thunder in the other writer's play, he got mad and shouted "They steal my thunder!" The phrase caught on, and now if you say that someone is stealing your thunder, it means they're getting all the praise, all the credit, or all the attention—for your idea, or your work.