And they do mean simple.

Radio stop commands in Poland can be conducted by anyone with just $30 in equipment to create commands using the correct radio frequency, Lukasz Olejnik, a cybersecurity researcher and consultant, told Wired. Hackers could have allegedly used simple tones that can be found in a European Union document, he said. The document says it lays out the “interoperability relating to the control-command and signaling substem of the trans-European high-speed rail system.”

But hackers wouldn’t necessarily need access to the document, according to Olejnik, who said the information is widely available online, particularly on YouTube. “It is three tonal messages sent consecutively. Once the radio equipment receives it, the locomotive goes to a halt.” He added: “Everybody could do this. Even teenagers trolling. The frequencies are known. The tones are known. The equipment is cheap.”