It obsoletes existing Barracks.
The development of the internal Combustion engine was a great advance of the Industrial Age. Relying on the volatility of new fuels made available by the Refining of oil, these compact engines developed great power. They were essential to the development of the Automobile and aircraft because previous engines were too bulky to be practical. In addition, the internal Combustion engine was extremely efficient in generating power for the fuel it consumed, which allowed vehicles, aircraft, and ships to travel reasonable distances.