A drive train which can be quickly changed out:
http://en.wikipedia.org/wiki/Powerpack_(drivetrain)
Power pack or powerpack is a term used to describe part of a modular powertrain. A powerpack module contains some type of engine (most frequently an internal combustion engine but other types, including electric motors, are possible) and may also contain a transmission and various supporting components. Powerpacks are used with certain types of industrial equipment designs, including vehicle designs such as forklifts and cherry picker lifts but also stationary equipment such as paint sprayers. Virtually all modern military tanks use them, and many other military vehicles as well.
The article goes on to note that one of the benefits is that the drivetrain can be quickly swapped out. Sounds familiar, but I just can't place my finger on who might have come up with such an idea.
