Hearts of Steel is a comic miniseries published by IDW in 2006. It was the first (and thus far the only) story in IDW's Evolutions series. It is set in an alternate Generation 1 continuity in which the Transformers' first contact with humanity is in the late 19th century, not the 20th.
|Hearts of Steel issues:|
|#1 | #2 | #3 | #4|
The story opens with the Transformers engaged in a battle on Ice Age Earth, and having adopted bizarre animal alternate forms as a result. When the climate cools, both sides retreat underground and enter a period of stasis, not awakening until the 19th Century. Soon both sides are making alliances with humans and utilizing the technology of the age to continue their war.
It is not possible to pin down exactly when this story takes place. The Civil War is not mentioned, so it is most likely after 1865. Mark Twain lived in San Francisco from 1864 to 1866, and he appears to live there in this story. However, he claims in Part 4, to have one daughter, which would place the story between 1872 (the birth of Susy Clemens) and 1874 (the birth of Clara Clemens). In this wiki, we've picked the arbitrary compromise date of 1867.
"Ice Age Wars"
The first few pages of this story show that the Transformers came to Earth immediately prior to an (unspecified) ice age, and fought a war there and then using dinosaurs, giant insects, and (possibly) other beast-forms as altmodes, before going into hibernation until the 19th century. Many of the main characters wear these distinctive spiky bodies for the first half of the story, before taking on vehicle-based forms. Where necessary in this wiki, this prelude to Hearts of Steel is referred to using the (unofficial) term "Ice Age Wars".
Most of the Transformers thus take on the form of fanciful locomotives and railway cars, rather than automobiles, etc. Some notable exceptions are Shockwave, who becomes an ironclad ship, Scourge, who transforms into a massive zeppelin, and Starscream, who becomes a biplane (which is a bit of an anachronism). Optimus Prime and Megatron do not appear, though concept drawings for them are included in the trade paperback.
It is interesting to note that the Transformers here almost seem to adopt not only the appearance of Earth's machines, but some level of its technology as well: locomotive-form Bumblebee is fueled by coal, Scourge flies using a real gasbag, and Ravage is sufficiently vulnerable that an exploding gas stove is enough to knock him out. Whether this is an actual story point or merely a stylistic choice is not clear.
- Hearts of Steel - alternate design mode sketches at IDW.