Trains may seem pretty mundane in the 21st century when compared with jet aircraft.

These days, trains play a small role in transporting Americans. Things are a bit flashier in Europe and Asia, where they're used for high-speed, comfortable travel.

This contrasts vividly with the previous century, when not just trains but armored trains were a vital piece of machinery in the two largest military conflicts of the era.

The armored train was first seen in the American Civil War, according to The Jamestown Foundation. But the battle-ready form of transportation came to prominence in World War I, when Russia used it as a means of defense during cross-country travel.

The trains were used by most of the European nations fighting in World War II: Poland took advantage of them extensively; Nazi Germany reacted and began using them; and the Russians kept their fleet up. Even Canada patrolled its west coast with one for a time in case of an invasion, according to Canada's Virtual Museum.

These trains were not just armored — they were heavily armed. Cannons, machine guns, anti-aircraft weapons, and even tanks were on board these moving walls of terror.

While the armored train could transport large amounts of firepower rapidly cross country, it also had quite a few drawbacks.

They were hardly stealthy. Their reliance on tracks not only limited where they could go, but it provided the enemy with an easy target: Sabotage the tracks, and you disable the train.

After World War II, automotive technology had caught up sufficiently to render the armored train obsolete. But these insane trains have left an indelible mark on history.

[An earlier version of this feature was written by Alex Davies and Travis Okulski.]