Multiple trains in poland have been stoped by an unauthorized radio transmission that included the russian anthem and a putin speech, causing massive delays.
Czarna seria wypadków na kolei. W nocy z piątku na sobotę doszło do włamania na częstotliwość polskich kolei i nieuprawnionego nadania sygnału radio-stop, który powoduje alarmowe zatrzymanie pociągów. Według nieoficjalnych doniesień w tle było słychać hymn Rosji.
That's correct. The signal is designed to be audible to humans over radio so there's no real encoding - either the train is able to automatically trigger the brakes upon receiving the signal or the operator has to do it manually after hearing the signal.
You don't even need an SDR, just a 20€ portable shortwave radio and a phone that plays the sounds of the signal over the mic.