Due to a temporary detour agreement BNSF was running trains on UP rails in Northern California and Oregon. On Saturday morning this workhorse was pulling the grade up to the summit at Grass Lake and as it went by a strong smell of diesel wafted trackside. The lead unit had sprung a fuel line leak as evidenced from the stream gushing down the panel behind the cab on the engineer's side, a major malfunction and potentially dangerous situation! We, as "concerned citizens" would do, called the UP line to warn them, and the train stopped just over the summit to isolate the unit.