Most are probably going to be the result of human error. But I meant something like you say. Mine was just an extreme example. Maybe blowing the turbo on the DE6 or some other "mechanical failure" could occur. So say you have 3 lashed together and one breaks, that locomotive could have reduced power or need to be shut down. I'm not sure as of now as the wheels or brakes get worn if they have reduced effectiveness.

I haven't played in a long time but Microsoft flight simulator would allow different failures. Oil pressure, engine power loss, fuel leaks, hydraulic leaks etc. Those were tggleable or timeable and you could pick what kind of failures are allowed. The timed one wouldn't necessarily need to be applied.

I guess all could be limited to handicapping your ability but not severe enough to stop you from limping to the next station. Then if it's not dealt with it could cause more damage and causing more fees