Western Railroad Discussion > DPU "dead spots"

How much of a problem are dead spots for radio reception by distributed power receivers? Do the companies out ground based repeaters in place?

I read that DPU loses signals around some curves, inside some tunnels, etc.

Posted from Android

It can go no comm intermittently depending on the terrain....and where it can present a problem with train handling there are repeaters that can be set up. If communication can't be retored relatively quickly the remote units go to idle and remain that way until commmunication is restored...This can be an issue where that power was needed...but these are the places where repeaters are provided too. Intermittent comm loss is just nothing especially if it the places it happens are well known and it is known that it won't affect your running. Weird issues that can't be explained aren't common and can usually be chalked up to some sort of equipment deficiency or failure. 



Edited 1 time(s). Last edit at 05/28/23 16:38 by engineerinvirginia.

In tunnels, what is used is called a "leaky antenna" in radio jargon.  It uses a repeater to amplify the radio signal.  The antenna itself is a wire stretched along the roof of the tunnel.  The antenna doesn't put out much signal strength in that configuration, but it is right above the antenna of the locomotive (or DPU's) while the train is in the tunnel.  Most DPU's are set up such that, if the DPU loses communication with the end end, it just throttles down to idle.  That can be a problem if it happens when the DPU's are needed for power, dynamic braking, or keeping slack bunched or stretched.  Also, the DPU, EOT, and HOTD radio signals are digital UHF.  The advantage of a digital data signal is that the signal (the "data packet") is of short duration and digital data signal will code and decode more "cleanly" than an analog or digital voice signal.  Compared to VHF signals, UHF is more line-of-sight, but its actual signal width is considerably less than an VHF signal, so it can pass cleanly through a smaller opening.   The UHF train telemetry signals are low power because they don't have to travel much farther than the length of the train.  This is done, in part, so that the train telemetry signals from one train don't interfere with train telemetry signals of another train that is miles away.

DPs will continue for 90 minutes in the last power notch command it received if comm is lost.  After 90 minutes without comm being restored the DP(s) will go to idle and cut out their brake valve.   It will remain in the last dynamic brake command indefinetly.  A service application of the automatic air brake is supposed to signal the DP(s) to go from power to idle.  An emergency automatic application is supposed to get the DP(s) in dynamics to go to idle.

For a few months we had problems in one location with both DP and EOT communications.  I believe it was eventually traced to interference from a new cell/radio tower that had been recently built.  Whatever it was, the problem went away.  A good thing as it was at a crew change location.   

Thanks!

Posted from Android

As mentioned by "57A26" the DP units will continue their last commanded throttle / dynamic brake position for a period of time in a comm loss state provided there is no change of brake pipe pressure. A brake pipe reduction will cause the DP units to throttle down to idle until communication is restored and the units are "normalized". 

As to the "leaky" antennas in tunnels, there is a flaw with this system in actual use. UP installed this system in the tunnels on SP's old Cascade Subdivision (Oakridge to Cascade Summit). When new it worked quite well but unfortunately as diesel locomotives work hard in high throttle positions part of the exhaust created is carbon. This carbon soot coats the antenna cables and reduces their effectiveness. Perhaps in the years I have been retired they have improved the system but before I retired the tunnels were a challenge at times. 

This is quite common even today. All along the coastline from Seattle to Everett communication is lost. It doesn’t pose much issue as it is relatively flat.

However, a little further up on the climb to Scenic, it can be a huge issue…especially in the tunnel while cresting the grade.

For over a year there was an issue in the Casacade tunnel where comm was lost with the DP…and like said before, the DP will remain in the last command it was given, until the air is set, then it will idle itself. This is a terrible scenario, especially when the head end of the train is going down hill and increasing in speed and the air needs to be set, and the comm is lost while the rear end is going uphill and the DP goes to idle after the air is set. It goes from being a very useful pusher to a boat anchor in a second, and can potentially rip the train in half. I’ve seen it.

On the east coast...I ran CSX trains to Baltimore. Before they installed repeaters in the Howard St Tunnel, I'd have to power-up my DPU to make sure it was in power before losing com going in the tunnel The problem was you needed to be in dynamic almost right up to the entrance of the tunnel because it was down a hill into the "bottom" and then right back up over the city. So that meant I'd have to come over the top of the last hill a few miles away way-under to let my leader control the train in dynamic coming down the hill into the bottom, but leave the DP in power (usually notch 2). Right before we'd go into the tunnel, I'd notch everything to 8. You'd be at a crawl on the other side of the tunnel and without comm, but at least you'd have all the power going. I'm blown away there weren't more issues with the whole ordeal than there were... a few separations and a couple of derailments hapened there. The repeaters helped, once they were installed.

There were still a bunch of places I ran that you would lose com, but they weren't as bad as the tunnel in Baltimore. DP technology is really neat, and when applied properly certainly has advantages. But they rolled it out here overnight with zero forethought, and we were expected to make-do. I hated their implementation of it....100% of all trains would be DP'd, even if you couldn't actually use the DP because of placement. But come hell or high water, your train would have to depart DP'd. Gotta meet those metrics.

57A26 Wrote:
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> DPs will continue for 90 minutes in the last power
> notch command it received if comm is lost.  After
> 90 minutes without comm being restored the DP(s)
> will go to idle and cut out their brake
> valve.   It will remain in the last dynamic
> brake command indefinetly.  A service application
> of the automatic air brake is supposed to signal
> the DP(s) to go from power to idle.  An emergency
> automatic application is supposed to get the DP(s)
> in dynamics to go to idle.
>
> For a few months we had problems in one location
> with both DP and EOT communications.  I believe
> it was eventually traced to interference from a
> new cell/radio tower that had been recently
> built.  Whatever it was, the problem went away. 
> A good thing as it was at a crew change
> location. 

In my area, when a train is parked on a siding, the DP units transmit a data packet once per minute.  I've assumed that was so the controlling locomotive computer would know that the DP's were still communicating. I just observed that again this morning.  Train was an intermodal--4 locos on head end, two-unit DP mid-train. with EOT on rear.  DPU set was doing the transmitting while sitting on the siding.

Lackawanna484 Wrote:
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> I read that DPU loses signals around some curves,
> inside some tunnels, etc.

I might have a video example of this in Montana, at Blossburg, west portal of the Mullan Tunnel. See ble692's comments linked, and scroll up for my video:

https://www.trainorders.com/discussion/read.php?1,2584767,2585931#2585931

If 3 minutes is too long, start at 2:00. ~2:17 is where the DPU "notches down" when it seems to re-establish communication. Most of the train is already going downhill, but with manned helpers in middle of the train.

Manned helpers in the middle of the train is an interesting approach.

Thanks for the example.

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Edited 1 time(s). Last edit at 05/31/23 05:14 by Lackawanna484.