• Trains hauled by multiple locomotives: how to synchronize?

  • General discussion about locomotives, rolling stock, and equipment
General discussion about locomotives, rolling stock, and equipment

Moderator: John_Perkowski

  by SouthernRailway
When a freight or passenger train is pulled by multiple locomotives, how does the operator ensure that all of the locomotives are traveling at exactly the same speed?

If I linked a few cars together and set them all to run at 65 mph, surely there would be slight discrepancies between their respective speeds, causing crashes.

Just curious. Thanks to everyone for answering all of my questions elsewhere.
  by DutchRailnut
Its just like a team of horses, not all are equal but as long as commands are right they all pull their share.
on locomotives each locomotive gives a certain percentage of power at each notch, and todays locomotive has 8 notches or throttle settings.
as long as all locomotives have same gearing, it does not matter what horsepower they got.
they each pull their share
  by Allen Hazen
(Don't be insulted if this is too obvious: I don't know how much you already know, and your question might be from a total beginner.)

(1) Locomotives typically don't have accelerator pedals: they have throttles that can be set at a limited number of settings ("notches"), typically eight in North American practice. When a train has multiple locomotives, they are electrically connected: there are "multiple unit control cables" as well as air-brake hoses connecting them. (These are located beside the couplers.) This connection allows the engineer in the leading locomotive to set the throttles in all the locomotives simultaneously, so that when they are pulling a train they will all be operating in the same throttle notch. (Some exceptions to this, but for simplicity I will ignore them.) Thus each locomotive is running at about the same fraction of its maximum power.

(2) The important thing here is that the throttle setting determines the power the locomotive is operating at, not its speed. (But see below.) The speed a locomotive goes is determined by the power and the amount of pull it has to exert on the train behind it... where the "train" includes any following locomotives. So when a train is being pulled be multiple locomotives, each is operating at a power that -- if it were uncoupled from the following train -- would make it go faster. So the first locomotive is "trying" to run away from the second (but can't because they are coupled together), the second locomotive is "trying"..., and the last locomotive is "trying" to run away from the cars of the train (but can't because they are coupled to it).

(3) Couplers are only so strong: if you suddenly went to full power from a standing stop, you might break the train in two. Part of the skill of being a locomotive engineer is knowing just how much power to use (= what notch to put the throttle in) at various speeds and different conditions.

(4) (This is what I referred to with the "see below" in paragraph 2.) The electronics in locomotive control systems have gotten steadily more sophisticated. (They started incorporating micro-processors in the 1980s: a modern diesel electric locomotive is basically a computer with diesel engine and traction motors as "peripherals." (Grin!)) On some it is possible for the engineer to set a desired speed. What this means is that the computer-- getting input from the speedometer-- automatically sets the power to what is needed. But the basic principle-- that what gets coordinated between the locomotives on a single train is power setting, and only indirectly speed-- is the same.

(If that's too simple-minded... Ask again: there are people here who know a LOT more than I do and can give detailed answers!)
  by AVR Mark
It should also be noted that in normal operation all locomotive wheels will slip a certain controlled amount. Since the late 1970s the amount of slippage is controlled by a computerized system. On some of the systems the actual locomotive speed is measured by an onboard radar system and compared with the speed that is measured mechanically from the wheels. The difference is speed of each axle is carefully controlled by the system to control the amount of wheel slip.

  by LocoCam
What the above all said, plus to clarify, all locos in consist don't each pull equally and this is alright. Since all are electric motors there is no mechanical connection between rail speed & prime mover speed. I don't even think they have to be same gear ratio between locos (though top speed of train would be limited to slowest loco). If one is pulling at 100Klbs and a second weaker one is pulling at 50klbs it still is 150klbs as far as the cars are concerned. More important to the engineer is that they "load" in a similar way (how each loco responds to power adjustments, mostly affects the ride in the cab). I commonly have both 6300hp units with 4400hp units in my consist.
  by Desertdweller
Good point, Lococam!

If there are locos in the consist that load at different speeds, there will be jerking within the consist if the throttle is opened too quickly.
The earlier GE units were especially bad for this. We used to say, "You open the throttle, pour yourself a cup of coffee, and about then it will start to move".

  by Allen Hazen
Slow loading comes up again and again as an imperfection in older GE locomotives.
At least the jerking WITHIN the locomotive consist would be avoided if consists were either all-GE or all-EMD. Do you know if any of the railroads with large fleets of GE "U-boats" (Santa Fe, SP) made any systematic effort to dispatch locomotive in "pure" lash-ups? Would have been a nuisance to arrange, but a terminal (at one or the other end of the Santa Fe main line, for example) dispatching dozens of trains each day should have been able to do it much of the time.
  by 10more years
Just a simple answer is that each locomotive has an electrical cable connection ( to control the power output) as well as an air hose connection (to control the brakes).
  by Passenger
SouthernRailway wrote:When a freight or passenger train is pulled by multiple locomotives, how does the operator ensure that all of the locomotives are traveling at exactly the same speed?

I ask the same question, but make that steam locomotives.

I know it wasn't as common back then, but there were situations going over the mountains.

How did they do it?
  by Allen Hazen
When steam locomotives were "double-headed" (or sometimes even "triple-headed"), each locomotive had its own engineer: running a steam locomotive was a highly skilled task, and each engineer had to know how to operate his locomotive so as to get the maximum "pull" without having the driving wheels slip.
  by ExCon90
I've always marveled at the way they did all that with no radios -- the only communication was with whistle signals and with the helper engineer (often on the rear) watching the needle on the air-brake pressure gauge to see what the lead engineer was doing with the air brakes and judging from that what he needed to do with the throttle. A helper engineer would routinely spend his entire workday doing that.
  by Allen Hazen
Back with diesels, one of the "exceptions" I ignored for simplicity earlier...
Suppose you've got the train up to track speed (=maximum speed that is safe on that stretch of track). You'll need some power to maintain the speed (friction in the bearings, air resistance, etc have to be counteracted), but maybe not as much power as you needed while accelerating (or climbing the hill).

Suppose your train has, say, three 3000 hp locomotives (reasonable for a mainline freight, at least in the eastern half of North America, in the 1970s and 1980s). Accelerating up to speed you'll use the full 9000 horsepower, but maybe you only need 6000 (arbitrary figure chosen to simplify the arithmetic of the example) to maintain speed. Simple: operate at a throttle "notch" at which the locomotives will be putting out 2/3 of their maximum power (3x2000=6000). Ah, but it turns out that if you want 6000 horsepower from three 3000 hp diesel engines of the sort used in locomotives, it is more fuel efficient to run two of them at full power (2x3000=6000) and reduce the other one to idle. (You'd save even more fuel, of course, turning one off entirely... but getting it started again might be a problem, so most railroads in the 1970s let their locomotives idle, even when parked over night!) In the 1960s, diesel fuel was cheap, but by the late 1970s railroad management started worrying about fuel efficiency. One approach was to install some extra gizmos in the locomotive control systems, so that an engineer could run some of the units on his train at full power while reducing others to idle.

Relevance to the original question: it turns out that trying to "synchronize" the locomotives isn't really necessary! It is entirely possible (and under some circumstances may even be a good idea) to have a few of the locomotives on the train doing all the work, with the others just being towed along for the ride!
  by GSC
Although matching power in multiple lashups would most likely be desired by dispatchers and the mechanical dept., there must have been times where seemingly incompatible locomotives would be MU'ed to provide the power needed.

On several occasions in the late 60s - early 70s, I saw such things as an ALCO RS3 and an EMD SD35 lashed up in sand train duty on the CNJ, both locos working hard. Someone I knew who worked on the CNJ back then told me that MUing a Fairbanks-Morse unit with anything else was frowned upon, although it did happen, but rarely.

During the mid-50s transition period on the Pennsy, it wasn't impossible to see a K4 steamer double-heading with an EMD E7 or a Baldwin Shark in passenger service.
  by Jtgshu
the best tool to ANY engineer, steam or diesel, is the one that he is sitting on....his OWN "hind end" - You can feel what the train is doing by what you are feeling in the seat. Its hard concept to explain, but very easy to understand once you can feel it and understand it.

Those doubleheaded steam locos would even more so be felt by the seat of their pants. Thats how to tell whats going on with the rest of the train and the other power, if you didn't hear a radio call or a whistle blow. If say the loco is pushing on the back - if the front coupler of the loco you are running gets the slack pulled out of it, and you feel the train tug your engine, that means (in a nutshell) that the train is being stretched, which more than likely means the engine on the front is pulling for power. Conversely, if the engine you are on in the back "runs into" the train and the front of the train seems to be slowing down as the engine you were on was going slightly faster, that basically means that the train is slowing down. Of course, grades and hills all play into this, but imagine a totally flat run to get to understand the concept.

The engineer should be able to see or feel what a train is going before seeing it in the speedo. Also, most (not all) engineers don't particularly use their gauges as much as you might think - especially the air gauges. Each train handles differently, so if you make a predescribed brake reduction at a certain location, one day it might be exactly what you want, the next day, not enough brakes, the next day too much brakes. You have an idea of what to do and where to do it, but you have to constantly adjust because of how the train is handling. Also, when running slow, or drilling (shifting cars around in a yard or an industry) most (again, can't say all) engineers don't even look at their "dash" but rather down at the ground next to the cab. After a while you get a really good feel for speed, and will only occasionally glace up at your speedo. It really doesn't matter if you have 1 loco or 10 locos.