These have probably been answered elsewhere, but in the interests of brevity I’ll ask them again…
Does anyone know the record number of diesel (not electric MU’s) locomotives that have been linked up as a head end unit? DPU’s don’t count- they aren’t physically coupled to the head end.
Is there a delay between the time a locomotive engineer changes control settings on the head end and the time the last unit in the MU consist responds?
Thanks
Erik
Erik, the largest number that I know of where all locomotives were online and powering is 16 on the old Chicago Great Western, all various models of EMD F-units. A few accidents and the development of six-axle locomotives with better wheel slip systems ended the large consists. I have seen light engine moves of 30 plus locomotives but most of them were isolated. The problem with large consists is not a time delay between the lead locomotive and the last ones, but rather whether the last locomotives will even receive the signal at all. Post 1960 locomotives are linked by a cable with 27 wires, 25 of which are used to control trailing locomotives, 2 are spare. The system works by either powering a wire with a 74 volt current or not. The problem with long consists is that resistance cause by corrosion and damage from pluging and unplugging the connectors means that trailing locomotives may only see a signal of 20 or so volts, not enough to trip the relays. Intermediate locomotives do not repeat the signal, for safety purposes the lead locomotive generates the signal and it passes down through the consist like a power bus. All railroads now have a limit of eight powered locomotives or less. The less is because newer locomotives generate enough tractive effort to break couplers with three or less locomotives, if the train is heavy enough.
Would you mind clarifying what you mean by this? I suspect that you are not referring to rough handling, but I would like to know a little more than what I do. Thanks.
The maximum are 36 powered axles on the head end of a train. Which would be 6-6axle units or 9-4 axle units.
That isnt always true … if you go to Al Krugs website you see that BNSF SD70MAC’s (6 axles units) are rated at 11 axles … so in that case you can only have 3 of those. The number of powered axles isnt always the exact number of axles.
It has to to with how much pull the combined axles will but on the drawbars. if you had 6 4400hp units online going up a hill at once … thats WAY more pull than the drawbars can handle.
So if each axle is rated as one then yes … but some big units are rated at more than one axle.
Just thought i’d throw that in.
Hmmm… doesn’t the signal from the front end of an MU consist get acted on, amplified and resent to the next unit in line?
No, that would be dangerous.If any locomotive misinterprets the signal it would lead all further locmotives wrong, this way only one would be wrong. Also there is no computer reading the signal, there is only a relay. If the voltage is strong enough the relay lifts, if not it doesn’t.
An AC4400CW is capable of pulling with 180,000 lbs of force on a coupler at speeds below about 8 mph, an ordinary freight car is capable of withstanding 300,000 lbs of force continuously, with a momentary ability of 360,000 lbs. So with two AC4400CWs on the headend of a mixed manifest train the Engineer has to be careful about operating the train at full throttle and low speeds. If the train is traveling at 20 mph, no problem. But if he is on a tough grade and the speed gets down around 9 mph, he is going to have to be very careful, if the speed falls further to say 8 mph, he is going to have to reduce the throttle to Run 7 even if it might lead to the train stalling. Even at 9 to 10 miles per hour any little thing like a momentary wheel slip could lead to a coupler breaking as the resulting slack will add to the regained pulling power of the locomotives.
Coal train cars are built to withstand 500,000 lbs of pulling force, with a momentary ability of 600,000 lbs. As you can see it is possible to operate with 3 AC4400CWs on the point of a coal train but the Engineer should be careful at low speed highpower. The Engineer might have to reduce the throttle to Run 7 if speed continues to drop below about 9 mph while in full throttle. If the pulling force limits are exceeded, the coupler, the drawbar, or the whole freight car may break.
This becomes even more of an issue once the locomotives have crested and start down a hill. The additional pull of gravity added to the tractive effort of the locomotives can easily overcome the breaking point of knuckles and/or drawbars.
I have various Penn Central employee timetables from 1969 in which the Special Instructions limited motive power consists to 24 powered axles. Employees were also barred from isolating indivividual motor circuits to meet this limitation. I will admit that this restriction is a bit dated, but it still seems valid based on other postings on this thread.
Jerry Pinkepank, in his article about MU control in “Trains” in the late 1960’s, mentioned a limit of four units for MU with air throttles and eight units for electric throttles. Loss of signal strength (either air or electric impulse) was the reason for the limits.
just to be explicit: the limit on powered axles has nothing to do with rough handling. It is pure and simple engineering – the last drawbar has a maximum tensile (pulling) strength, and as noted it is quite possible to exceed that number, if you have too many powered axles. Which is one of the main reasons for DPU, pusher helpers, and so on.
A ham-fisted engineer can bust a drawbar with four powered axles, if he’s having a particularly off day.
Y’all got me to thinking about handling freight trains in the “old days”; I wonder if a Big Boy engineer could have yanked a draw bar out as easily?
Interesting responses. I understand the limitations of coupler strength. I even understand why you don’t want too many units linked together if the commands sent by the head end aren’t strong enough to lift a relay. I’m somewhat surprised that with all the computer technology aboard a modern locomotive that MU signals are still bursts of raw electricity- not digital commands that are picked up, amplified, and relayed to the next unit.
Of course, if the system isn’t broken- why fix it?
Or if he’s just a real lousy engineer.
I am surprised that the GE and EMD arent installing a new distributed power network on the locomotives, one that would be backward compatable with older units, but allow newer units a more sophiscated control scheme…since enven a Catapillar Tractor and Bulldozer uses network devices onboard nowadays, it would seem an easy task to install a simple plug with 4 wires in addition to the already present 27 wire device…keep the modern ones together, and have the last unit relay the commands via the older technology to the “Legacy” units in the consist. Heck, you could even have adaptave software that learns the responses of the older units to fine tune the control,and eliminate some of the differences in response you would get…
[Of course, if the system isn’t broken- why fix it?]
And it wont get fixed unless someone demonstrates there would be something to gain with the increased cost and complexity of such a system…unlike most of us who own a computer just because they are “neat”, the benefits have to go much farther than the “Gee-Whiz” quotient to but that kind of dollars into a system.
The technology exists to reduce the control cable to what amounts to a phone cord. The problem, as JSG points out, is backward compatibility. Right now, you can hook up a 50-year-old locomotive to one right off the assembly line, and they’ll talk to each other.
Running a parallel system might allow you to monitor the old system with the new, and possibly do some “repeater” work, since everything could be verified via the new system.
Of course, with 2-4 units doing what would take 8 or more “back then,” you’re not really fixing something that needs fixing.
It might be better to go to a totally wireless solution - why bother with the MU cables at all? The telemetry unit would connect to the existing MU set-up. Encryption and unit addressing (very much like what the DCC folks do in the model RR realm) would mean that an engineer could control a locomotive consist of a length dictated by signal strength (not that the current limitations are much different there), and possibly even DPUs, just by doing a few keystrokes on the onboard computer.
Age of the locomotive would be of no consequence, since the system could be set up to run as a remote, connected to the MU cable (or another connection inside the cab) very much as is done with FREDs now (as in, take wireless control box 1234 and hook it up to the third locomotive in the consist).
Probably, but of course the T.E. of a Big Boy is less than one modern diesel locomotive.
No burst involved, the signal is continuous, if the signal on any wire is lost the power level changes.
As to why it hasn’t changed, backward compatibility, the new system must offer enough benefits to offset the loss of compatability with older locomotives that will last another 20 - 30 years.
Erik, this is probably the best website for learning about how to handle trains and the forces that act upon them.
Al Krug’s Homepage
Thanks, Beau… I tagged Mr. Krugs website a long time ago, and you are right- it is a valuable reference. I noticed he hasn’t been up lately with anything new… has he moved on to something else?
I am now curious about push pull units on passenger trains. Seems to me that when the locomotive is in “push” mode, the engineer is actually wedged up in a cab in a modified passenger car at the rear of the train. This would seem to indicate that there is a different control system in use, somehow… or am I out in left field? Would not the intervening three, four, or five passenger cars also degrade a command signal from the passenger car “cab” to the locomotive?
And because I am nothing but an overgrown kid with a box of matches and a five gallon can of gas (which I will now explode on this very thread- stand back!) how did the MU systems work on electrically powered trains? I remember watching commuter trains out of New Haven many years back and it seemed that every third car had a pantograph raised. Was the lead car the only one pulling the entire consist?
Thanks y’all!
Only the control systems are mu’d in most cases . There is such a thing as a bus jumper that would negate the neccesity of raising all the pans but it is easier to collect the power seperatly so as not to overload the only current collector in use .
Randy
By the way, I wrote Al Krug. He is still working for the railroad and is on yard duty. He does not take photos and post them because he figures he’s gotten the best shots already. He does have a great web site and a lot of plain common sense about railroading that is easy to read and understandable.