Transport: So That's a SBU

Don't call it a FRED! A SBU sat on the coupler of the last car of Canadian Pacific train #240 as it crossed the Humber River in Toronto, Ontario on 25-February-2009

TORONTO, ONTARIO - Every child knows what should be at the end of a train--a caboose. Unfortunately, since the crew size on most mainline trains was reduced to two people in the 1980's, that hasn't been the case. With the engineer and the conductor in the locomotive, there was no need for the caboose, and it was replaced by a device to monitor the air brake pressure at the end of the train.

For most of my life, I've thought this device was properly called a FRED, or Flashing Rear End Device. (Some railroaders claimed the "F" stood for something else easily guessed with a colorful imagination but not printable here.) It served a number of purposes, a marker at the end of the train so that it would be visible to following trains, a monitor of air brake pressure at the end of the train transmitted by radio to the locomotive so the crew knew whether their train was intact, and in some cases it was capable of setting the brakes from the end of the train when requested by the crew in an emergency.

Up in the locomotive, there had to be a device to receive the signal from the FRED, and these receivers came to be called "Mary"s. Mary didn't seem to stand for anything; someone just thought it would be amusing and the name seemed to stick. When trains departed major terminals, the crew would often be asked for their "FRED and Mary numbers" to keep track of the devices. Apparently in some regions, Wilma was used instead of Mary in a nod to the Flintstones, but I have personally never heard that term used.

FRED and Mary were rarely used as official names. The FRED was sometimes called a EOTD, or End Of Train Device, with the "D" later dropped. On Burlington Northern "wheel" reports for trains in the early 1990's, the FRED came to be labeled as the REEOT (Rear End End of Train device) and the Mary came to labeled as the HEEOT (Head End End of Train device). Having an "End of Train" device at "Head End" of the train apparently didn't strike the railroad as being the least bit ironic, but the terminology had clearly become messy. In time, the HEEOT became a HOT, or "Head Of Train" device, which is perhaps a bit better.

Since moving to Canda, I have heard Canadian Pacific crews and Rail Traffic Controllers refer to a "SBU". It seemed like they were talking about a FRED, and this prompted a bit of research. It turns out that the Association of American Railroads, of which virtually every railroad in the United States and Canada is a member, adopted in 1999 a new terminology for these devices based on the terminology that originated here in Canada. The device on the rear of the train is now called a SBU, or Sense and Brake Unit (though there are two different device codes based on whether it can apply the brakes). The name at the head of the train varies based on function. If it just displays information, then it is a Receiver Display Unit (RDU). If it can control the brakes at the rear as well, then it is a Communications Display Unit (CDU). Sometimes the are two devices, the Communications and Logic Unit (CLU) to receive the information, and the Information and Display Unit (IDU) to display it. The whole thing seemed very Canadian to me, quite accurately describing the equipment, but with names that are almost completely non-memorable.

Thus, it's not surprising that in most places in the US, one still hears about "FRED and Mary numbers," not SBU and RDU numbers. But, the next time you see a train, if you want to follow AAR practice, remember that not only is there not a caboose on the end of a train, there isn't really a FRED either. It's a SBU.