Railroad DTMF

[doriboni]

This topic is interesting, I had never been interested in the subject before.

I have a question though: How is safety ensured in radio systems that use analog DTMF?

How do they prevent anyone from sending DTMF commands by copying and re-emitting the audio signal sent before?

 

[wa8pyr]

For critical applications like track switches etc, railroads typically have a rule in place that requires a train crew to enter the DTMF command to place the switch in the desired position, even if they know it to already be in that position. This would negate an invalid command sent by a bad actor.

Switches etc also have voice announcements which state the position of the switch after a command is given, for an extra measure of safety.

Finally, devices like this are extremely short range, meaning a bad actor would have to be really, really close in order to actually do something. Most of these kinds of devices are very remote, often out in the middle of nowhere, and getting close to them except by rail is tough.

 

[doriboni]

Thank you for this information.

There is more and more talk about China preparing to attack American infrastructure. Could Chinese agents attack the entire US DTMF network?

https://www.reuters.com/sustainability/climate-energy/ghost-machine-rogue-communication-devices-found-chinese-inverters-2025-05-14/

Rogue communication devices found in Chinese solar power inverters​

• Rogue communication devices found in Chinese solar inverters
• Undocumented cellular radios also found in Chinese batteries
• U.S. says continually assesses risk with emerging technology

 

[AK9R]

DTMF network?

These DTMF-controlled switches are not part of a network. They are standalone. They are DTMF-controlled because they are not part of any centralized or remote-controlled system.

Most railroads operate under a rule that says you must visually verify the position of the switch points before you move through the switch. Also, switches have a visual indicator of switch position using either flags (aluminum or steel) of different colors or lights. To my knowledge, DTMF-controlled switches are used in areas where the track speed is low enough to stop a train if a member of the crew sees that the switch is misaligned.

One other thing, I suspect that these switches have lock-out features so that if the switch is occupied by a locomotive or car or the switch has been occupied in the past few minutes, the switch will not operate even when commanded.

 

[exkalibur]

The worst someone could do is cause an inconvenience. While DTMF does control some safety critical things, as others have said, there must be manual verification.

 

[doriboni]

Okay, thank you. This reassures me.