Need some insight from people who have driven locomotives


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k4kfh

Member
I am trying to write a piece of software to make a model train controllable in the same way that a real diesel locomotive is. Obviously I can't emulate a real diesel engine, and a functional air brake system on 1:87 scale, and certainly not using only software. So I need some insight from people who have driven locomotives. NO SOFTWARE KNOWLEDGE IS NEEDED, if you have driven a locomotive please don't think you don't have "the knowledge" to answer this.

One of the most obvious things I'll have to replicate is the behavior of a notched throttle. Having never driven a train before, I simply assumed that something like:

(the notch/8) * 100

Would give me my speed as a percentage of my max speed. However in fiddling in RailWorks train simulator 2015 (closest I could get to the real thing here at home haha) I found this not to be the case. In RailWorks, the notch of the throttle seems to only control how quickly the train accelerates to it's max speed. (I did test this with no cars, so that may affect this, but I'm trying to keep it simple by not taking load weight into account just yet) For example, if I have an F7 unit idling, and I put it in forward, and I take all the brakes off, and I put the throttle notch on 1, then it starts moving. The speedometer slowly, but surely, climbs up, and if I wait 2-3 minutes I'm at 50mph without having touched anything. Is this accurate to how a locomotive really behaves?

Once I figure out the notching system and how to get the speed of the train based on the notch of the prime mover, then I'll jump into braking. For now I need to get a notch system that's as accurate as it can be (within reason, obviously there is a point where "good enough is good enough")
 
k4kfh said:
I am trying to write a piece of software to make a model train controllable in the same way that a real diesel locomotive is. ... One of the most obvious things I'll have to replicate is the behavior of a notched throttle. Having never driven a train before, I simply assumed that something like:

(the notch/8) * 100

Would give me my speed as a percentage of my max speed.
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Totally wrong assumption. Generally the throttle position has little to do with the speed of the train. That is a model railroader thing.

However in fiddling in RailWorks train simulator 2015 (closest I could get to the real thing here at home haha) I found this not to be the case. In RailWorks, the notch of the throttle seems to only control how quickly the train accelerates to it's max speed. (I did test this with no cars, so that may affect this, but I'm trying to keep it simple by not taking load weight into account just yet) For example, if I have an F7 unit idling, and I put it in forward, and I take all the brakes off, and I put the throttle notch on 1, then it starts moving. The speedometer slowly, but surely, climbs up, and if I wait 2-3 minutes I'm at 50mph without having touched anything. Is this accurate to how a locomotive really behaves?
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Yes, the RailWorks is right. The throttle only controls how much power is being applied. The weight of the train, momentum, grade, curves, track conditions, etc. determine the speed. That is why there is such a big emphasis in model railroad throttles (physical and computer) concerning momentum simulation. There is a big difference in the power of the locomotive also. A GP9 at notch 6 will behave differently than an SD45, and differently than an RS4

And just a caveat, I am not a locomotive driver, but know simple physics, locomotive design, and I work for a company that works with railroad simulation. One of our products is the simulators the railroads use to train their engineers with.
 
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So the throttle controls how quickly the train gains speed, not how much speed it gains, but is influenced by other factors like brakes, track grade, weight of the train, etc?

Hmmmm...can you think of a good way to simulate this? Obviously I'm going to have some configuration parameters for different locomotives, but I'm not even sure how to approach this from a math perspective (I'm a sophomore, the last math class I had was Geometry I, I haven't taken any physics yet, so forgive me if I ask dumb questions)

Is there some kind of a multiplication system I could use to look at the notch and find how much the speed should increase over a given number of seconds? Maybe using a line graph or something, with a set slope? Or would it be curved? Again sorry for dumb questions, thank you for your help

EDIT:

Maybe it would be better if I understood what everything in the locomotive does. I know you have a throttle with notches (usually 8, right?), and you have the locomotive brake, and the train brake. I have also heard of "automatic brakes" which from what I understand is the synonymous with "train brake". Is that all correct?

Now, I have a weird question. WHY does the throttle only change how quickly the train gains speed? The throttle adjusts how fast the prime mover turns the generator(s), right? So it changes how much electricity is going to the traction motors, which should change the speed at which they turn, right? (I know this logic is flawed, I'm not trying to call you wrong, I'm trying to see if you can help me understand where I'm understanding it wrong)
 
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I'm doing this to work with DCC locomotives, but not any particular decoder type or scale, since it will be built on top of JMRI. Whatever layouts JMRI will interface with, this program will interface with (if I can ever figure out the simulation system haha)
 
If you're ever in OKC, they have an 0-4-0 Porter you can drive. I think the last "track time" was $350. You could feel how at least that little porter reacts to throttle. Maybe other places do something similar.
 
I was trying to use RailWorks for that, but I wasn't sure how accurate RailWorks is. (also thanks for the heads up about the place in OKC, but I'm trying to simulate diesel locomotive controls, I decided steam would be too complex as a starting project) I've heard some of the train simulators are really accurate, and some are not so accurate, just never heard which ones fall into which category. Can someone shed some light on this? If RailWorks is pretty close to the real thing, I can do some fiddling and test things in there, but if RailWorks is only somewhat accurate, I don't want to waste time trying to base my software on RailWorks.
 
k4kfh said:
can you think of a good way to simulate this? Obviously I'm going to have some configuration parameters for different locomotives, but I'm not even sure how to approach this from a math perspective (I'm a sophomore, the last math class I had was Geometry I, I haven't taken any physics yet, so forgive me if I ask dumb questions)...
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That makes things hard. I presume you have also not mastered how to drive a car. Believe it or not driving an auto with a manual transmission would help you to understand.... To really simulate it accurately a physics class would be a good place to start. esp. Newton's laws of motion. I had physics in 7th, 9th, and 12th grades. I forget others did not. Not meaning to scare you away but you need realize that this is the exact reason Newton had to invent Calculus. Acceleration & momentum are continuous functions that require calculus to calculate properly. Then the next problem is that computers are not continuous (stupid digital devices) so there are special techniques used in computer science to estimate the formulas. I just did a quick search on the internet and couldn't come up with anything for beginners. I found some Euler-Newton formulas written in FORTRAN, but not quite what you need. Just know there is a reason some of the people on our staff who write the simulators have PhDs in physics, math, and/or computer science.

All that doesn't mean (as you suggested) you couldn't just use some sort of curve to fake it with. That is what all the DCC systems I've seen do. You could also just use two or more curves. One for max speed and another for how much time to get there. Adjust the curves for the loco type. It all depends on how realistic you want it to be. In DCC you could read the feedback from some of the decoders to determine how hard the loco is working. The harder it is pulling the more time it would take to get up to speed, or the reverse how much to slow down because of the increased load (like going up hill).

Maybe it would be better if I understood what everything in the locomotive does. I know you have a throttle with notches (usually 8, right?), and you have the locomotive brake, and the train brake. I have also heard of "automatic brakes" which from what I understand is the synonymous with "train brake". Is that all correct?
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I don't know what an automatic brake would be unless it is just that the brakes will come on if a brake line is broken and lets the air out of the system. That really has nothing to do with the loco. The locomotive has regular brakes just like an automobile (shoes or disks), it has dynamic brakes where the drive motors are turned into generators which slows it down, and finally it has a control for the brakes on the train. I think it depends on the loco if the regular brakes on the loco and train have independent controls or not, but there I am just guessing. I don't recall them being separate on the simulator I've played with (GP9 type).

Now, I have a weird question. WHY does the throttle only change how quickly the train gains speed? The throttle adjusts how fast the prime mover turns the generator(s), right? So it changes how much electricity is going to the traction motors, which should change the speed at which they turn, right?
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Almost, the throttle adjusts how much electricity is supplied to the motors (which is different for DC and AC locomotives). When the motor needs more power (or faster frequency power in the case of AC) the prime mover will automatically power up to generate it. The heavier the load the higher the diesels will crank up. "Why?" is actually the easiest thing to answer. A train is very heavy, it takes a lot of energy to get it to move and keep it moving. The higher the throttle the more energy the locomotive is putting into the train, so things change quicker.

hope that helps.
 
Oh, so the throttle is more like a potentiometer between the generator and traction motors, the diesel engine just adjusts its speed to accommodate? How did they do that on the old locomotives before they had computers? (Just curious)
 
k4kfh said:
Oh, so the throttle is more like a potentiometer between the generator and traction motors, the diesel engine just adjusts its speed to accommodate? How did they do that on the old locomotives before they had computers? (Just curious)
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That is an over simplification but is an OK working model for conceptualizing it. One does not need a computer do know how much current is being demanded by an electrical appliance. I don't know how they did it exactly but my first thought would be a series of relays in series with the motor. The more current it pulls the more relays snap each one increasing the fuel to the diesel. One can immediately see the flaw in that idea, so I'm pretty certain they were using something more sophisticated than that.

Computers don't really do anything that couldn't be done before they existed, they just made all those things changeable (programmable), much smaller, quicker and probably most importantly, cheaper.
 
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The throttle of a diesel locomotive controls the output of the alternator/generator by varying engine RPM (for the most part - the RPM can be the same for adjacent throttle notches). Speed and acceleration of the train has more to do with track grade and train load than throttle position.

Some notes on train brakes: A typical diesel locomotive is equipped with two brake valve handles: the independent brake and the automatic brake. The independent brake works on the lead locomotive and all the locomotives MUed to it. The automatic brake works on the locomotive consist and the rest of the train.

The automatic brake valve keeps the brakes on all locomotives and cars released when the brake system is fully charged. As the brake handle is moved into the service position, air pressure in the brake pipe is reduced which signals the control valve on each car or locomotive to apply the brakes. Moving the automatic brake handle to the emergency position causes air pressure to reduce quickly to zero which fully applies the brakes. The same thing happens when the train separates - air pressure in the brake pipe reduces quickly to zero and the brakes apply fully.

In freight mode, the automatic brake can be applied by making reductions in brake pipe pressure varying from a minimum set (6-8 pounds reduction) to full service (26 pounds reduction), which is called the service position. These numbers assume a starting/normal pressure of 90 pounds, which is common for freight carriers.

Here is one of the key points of the automatic brake in freight mode: once the brakes are applied, to get a reduction in braking force the brakes must be fully released. In other words, if you make a minimum reduction and it's not enough to slow the train, then you make a ten pound reduction and it's too much, you can't go back to the minimum reduction without first releasing the brakes. This can take some time, so it's important to plan ahead and to know the territory well to keep the train under control.

Anytime the automatic brake is used, the locomotive brakes also apply. Since locomotives are typically much heavier than the railcars they pull and since there are many more brake cylinders on locomotives than railcars, locomotives generate a lot of braking force. When the automatic brake is used, the locomotive brakes must be released or bailed off to avoid the train bucking the locomotives or causing the locomotives to slide. Therefore, the independent brake is used for light locomotive movements and switching operations but not during the operation of the train.

Another type of brake that locomotives are commonly equipped with now but wasn't always so common is the dynamic brake. Dynamic brakes work by reversing the polarity of the traction motors and using the momentum of the train to generate energy through the traction motors which is expended in the form of heat energy. To accomplish this, the train slack must be bunched to avoid a run-in, which can damage freight or even cause a train separation. Once the slack is bunched the dynamic brake handle - which looks like and more or less works like the throttle - can be moved though positions 1 through 8. Unlike the throttle, there are no notches. The dynamic brake instead works like a rheostat.

Back to the throttle... as I said the train's speed has more to do with terrain and load than throttle position. If the train is on level track - which is rare, in my area anyway - it doesn't take much throttle to move the train even if it's heavy. Making it accelerate is another matter. The heavier the train the more power it will take to accelerate. Starting a train on a hillside takes some effort. Depending on the steepness of the hill and weight of the train you may need to get pretty deep into the throttle before you begin moving. Going downhill most often doesn't require any throttle, but if the locomotives are not equipped with dynamic brakes or they are inoperative, some throttle may be required to keep the slack and train speed under control while the train rolls downhill with the brakes applied. Keeping the slack stretched is especially important if there is a caboose on the rear of the train, since the crew doesn't want to be thrown about.

In all the territories I've operated, the terrain undulates far more often than it is level or is straight up or downhill. Many times portions of the train will be going uphill while some are going downhill. It's important to know which part is where and what it's doing. Once the train is moving, it is the job of the engineer to manage the train's momentum and speed while keeping the slack under control to avoid damage to freight. There are times when the train is drifting uphill to a slower section of track or acclerating downhill to a faster section. The point is use of the throttle is specific to the conditions of the territory.

Since model train layouts don't often have the kind of grades the real thing does it's going to be quite a challenge to simulate what's actually going on. My advice would be to spend some time observing trains at different locations. Look for trains under load climbing hills, starting from a stop, switching in yards, going downhill using dynamic brakes and so on. Take notes, make recordings and try to identify the conditions the train is operating under. And of course, keep asking questions.
 
Two different things. The throttle controls how fast the diesel engine (the motor in the hood) runs. The diesel engine creates electricity and that runs electric motors on the axles. The diesel engine and the axles are not directly connected.

A train with 1000 tons in run 4 on the level will be doing 60 mph. A train with 10000 tons in run 8 on a hill might be doing 10 mph. The speed depends on the gearing the weight of the train, the grade, what the transition is set at, the horsepower of the engine.
 
OldGuyHO said:
If you're ever in OKC, they have an 0-4-0 Porter you can drive. I think the last "track time" was $350. You could feel how at least that little porter reacts to throttle. Maybe other places do something similar.
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The Western Pacific Museum in Portola CA has some larger diesels that can be "rented". Other RR museums across the country may have them as well.
 
It's funny recalling my time as an engineer on the Penn Central the FL9 (Just a EMD F7 with electric capability added) were the most common engine for passenger service to and from Grand Central Station. With in a few seconds of leaving a station the throttle would be in the 8 notch and the sand would be locked in the on position. That was the only way you could keep on time with the schedule we had to run by. I only worked local freight but I can remember Rs3's had a very quick throttle response. P3's electric locomotives had power to spare and could move a 13 car passenger train with little effort.

George
 
dave1905 said:
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A train with 1000 tons in run 4 on the level will be doing 60 mph. A train with 10000 tons in run 8 on a hill might be doing 10 mph. The speed depends on the gearing the weight of the train, the grade, what the transition is set at, the horsepower of the engine.
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Reminds of the times chasing trains on the old Saluda Grade in NC. The loco's would be in Run 8, and would be moving at the rate of a fast walk, with only 20-25 cars behind them. These were 3-4 SD-60's or 70's and most trains had to triple the hill with most consists. It was called the Saluda shuffle.

I sure do miss those trips.
 
dave1905 said:
A train with 1000 tons in run 4 on the level will be doing 60 mph. A train with 10000 tons in run 8 on a hill might be doing 10 mph. The speed depends on the gearing the weight of the train, the grade, what the transition is set at, the horsepower of the engine.
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That is how the C&S ruined the E5 units. A high gear passenger loco (112mph as I recall) creeping over monument hill with a heavy freight at 15mph. The load compensator couldn't adjust enough and all the traction motors fried.
 
Iron Horseman said:
That is how the C&S ruined the E5 units. A high gear passenger loco (112mph as I recall) creeping over monument hill with a heavy freight at 15mph. The load compensator couldn't adjust enough and all the traction motors fried.
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Does that qualify as an "Ouch!", or an "Oops!"?

Couldn't the trainmen smell that something was going wrong?
 
on a differrent note, on the old interurbans, you had 4 motors, 2 on each truck directly geared.
you dont have a potentiometered style controller, you have a controller, this can put the motors in series or parallel and some resistor banks (big resistors) to produce power levels. You "notch" the controller to the first notch, you may have 2 sets of motors in series with resistors to give you the lowest speed, notch up one may drop the resistors out, next up the motors are in parallel with resistors, notch up again all motors in parallel for top speed. No inbetween speeds. The motorman will notch back and forth for differrent controlling speeds or just drop the power and coast at the desired speed. Trains roll very well and won't lose speed very fast coasting.
 
I grok it! It's like driving an old International Harvester Tractor, the throttle SET has not much to do with how FAST you want to go! LOL!
 
RCH did a great job of describing both braking and throttle use. When I was a kid, I had the opportunity to have spent countless hours riding with relatives in the cabs of steam, diesel and electric locomotives with them working on both the Norther Pacific and Milwaukee road. I can remember them trying to describe what RCH did a great job of doing, but one thing that really stands out about their description is that you "gain a feel for it". Time behind the throttle was extremely important and braking in the mountainous terrain of western Montana required a lot of experience also. They always had to think miles ahead of where they were.
 
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