A diesel engine runs away due to an additional fuel source getting into the engine.
A diesel engine run away is when the diesel engine draws extra fuel from an unintended source and then over speeds at higher and higher RPM producing up to 10 times the engine's rated output until the diesel engine is destroyed by mechanical failure or bearing seizure through lack of lubrication.
Sometimes the diesel engine that is running away can be stopped if you can shut off the air supply to the engine or find the unintended fuel source and shut the fuel source off to the engine.
Other times there's not much that can be done but to just let the diesel engine continue to run away until it shuts off due to mechanical failure.
I once had a tractor that had the diesel run away and it finally shut down after an hour of running and destroying the engine after it locked up.
Sometimes a fire can also happen when a diesel engine runs away and other times the engine just runs until it's shut down on it's own or when someone is able to get it shut down.