If prototype trains had speed curves, what would they look like?

Mr. Trainiac Dec 31, 2020

  1. Mr. Trainiac

    Mr. Trainiac TrainBoard Member

    I just got a new SPROG 3, which means it is finally time to speed match, program, consist, and do everything else to my locomotives. I am looking at setting up some speed curves, but the default settings are just linear lines between the Vstart and Vmax CVs. If real trains had DCC decoders and speed curves, what would they look like? Would they be a straight line, or would they have some kind of curve, logarithmic or otherwise?

    There is still a lot to explore with momentum and BEMF, but does anybody program their speed curves with a low bottom end to simulate starting a heavy train, or do you use momentum CVs for that?

    I have heard people talk about having their yard power and road power having different curves. Most of my stuff is modern road power, so I think 70 scale mph as Vmax is good. It will look good on fast intermodals, but it isn't over the maximum speed of the prototype.

    This might be kind of a weird question, so I hope I'm not overthinking this.
  2. acptulsa

    acptulsa TrainBoard Member

    Steam or diesel? What kind of steam or diesel? Passenger, drag freight, fast freight or local? What era? A major railroad or a minor one?

    The "speed curve" of a real train is established by the laws of physics in accordance with the lengths the railroads choose to use to overcome the laws of physics with the technology they have that day. So, there is no one simple answer to your question.

    Believe it or not, the "speed curves" (I put that in quotes because I can't imagine a real railroad ever using the term) of steamers and electrics/diesel-electrics are hugely different. Anything with electric motors geared to every axle will more easily start tonnage moving from a standstill. But Lucius Beebe said a steamer would win a drag race hauling tonnage up to speed against an electric every time. And he would know, he was there. Is one of those curves parabolic and the other logarhythmic? Or are there so many variables that the equation that describes it (if you can find it) would fill your whiteboard?

    There's a reason we eyeball it. Sometimes trial and error really is the quick and easy way. The only answer I can give you is, good engineers try to make it as smooth a curve as possible. 1:1 engineers do that because they'll lose traction if they don't; model engineers do it because it looks like what the 1:1 engineers do.

    Oh, and I have more bad news for the DCC fans. No matter what perfect speed curve you come up with, it'll be wrong sooner or later. You'll use that engine to haul one single track geometry car, and it'll still look like it's straining against a hundred car drag. Sorry.
    Last edited: Dec 31, 2020
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  3. bigGG1fan

    bigGG1fan TrainBoard Member

    As a newbie to DCC and all these possibilities, I can give you my brief observations on my layout. More experienced/prototype-based modelers may have more concrete advice, and bear in mind I like to see my trains run, throttle up/down on the grades, and break apart/assemble passenger trains.

    Speed matching is a must if you want to run MU. I haven't played with BEMF yet, other than to turn it off so I don't get the push-pull response on my grades while I'm playing. I will spend more time on momentum and such in the coming year. But as was pointed out earlier, there's no difference in the engine's programmed response curve regardless of whether you're hauling one car or twenty.

    I probably won't set up permanent consists for most of my equipment. I have accumulated a ton of engines, and programming all the possible combinations (even if I restrict it to within the same road name) is time spent not running trains.

    As for setting prototypical maximum speeds, I've been speed checking my trains and engines for the last month. I find that prototype speed *looks* slow. At the same time, I don't want to get entirely unrealistic, or worse, have my 8 year old niece turn it up to 100% and send my COLA flying onto the concrete floor below. I'm thinking between 10-20% over prototype maximum will give the appearance I want.

    But always remember rule one: It's YOUR railroad. You can do what you darn well please.

    Sent from my SM-N950U1 using Tapatalk
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  4. Mr. Trainiac

    Mr. Trainiac TrainBoard Member

    I think I am in your boat with consisting. I am not a big fan of the advanced consist addresses, but WiThrottle supports universal consisting where you basically just add addresses to the active one. The NCE systems seem to work similarly.

    I have two locomotives speedmatched right now, and I have considered getting the Accutrack speedometer to aid in this too. Right now I am following the 'golden locomotive' system where everything is matched to one baseline locomotive. I don't really have that many locomotives when it comes down to it, especially locomotives that would prototypically be in a consist together.

    The hard part is dealing with different brands and their acceleration and motor control. I have deacceleration matched, but one has a higher starting voltage then the other, so getting them to start at the same time is more difficult.
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  5. BigJake

    BigJake TrainBoard Member

    NCE uses Advanced Consisting; they just have you set it up and run it like it was universal consisting.

    And UP has proven that ANY locomotive combo can be prototypical... They ran their Big Boy steam engine all around the country, in a consist (at least visually) with a modern diesel!
  6. Dave1905

    Dave1905 TrainBoard Member

    Real railroads don't work that way. An engine can be in run 8 at maximum power and be doing 10 mph or in idle doing 70 mph.
  7. rch

    rch TrainBoard Member

    I think it's important to make the locomotives work together more than anything else. How you manipulate the throttle will be the way you convey the load the locomotives are pulling.

    I use NCE DA-SR and D13J decoders for most of my non-sound locomotives. In order to make them respond and stop somewhat close to the way the prototype handles I set CV 3 to 30 and CV 4 to 30. Of course I'll tweak individual locomotives so they match each other, but this is my baseline. This makes them take some time to accelerate and decelerate but it's not too much. Sound locomotives get set up to match.

    Even light power on level ground when you fully apply the independent from 10 MPH you'll travel about an engine length before stopping. It's much more than that with a couple thousand tons behind you, but then you're usually using the train air to stop (unless you're a yard job switching cars off air). In order to mimic that effect you just have to use throttle modulation. As I'm sure you can imagine grades can dramatically affect the train acceleration and deceleration, even grades as small as 0.5%.

    As far as sound is concerned you might think about controlling the engine or dynamic brake sound separately from the locomotive speed. As I'm sure you're aware the throttle setting is not married to the speed of the locomotive. Depending on the territory you can start the train rolling in idle or even in dynamics and other locations require full throttle to start. And the era of the equipment makes a difference, too.

    Older locomotives - such as SD40-2s and C30-7s - had less effective dynamic brakes so controlling the train speed and slack was accomplished with train air. In those cases you are likely to use throttle to drag the train to a stop with air set. Not only that but if you had an occupied caboose on the rear you needed to control the slack or you'd toss your conductor and brakeman around, so dragging against brakes was common.

    Modern locomotives - Dash 9s and SD70s and their AC counterparts - have very effective dynamic brakes by comparison so using the dynamics to slow or stop the train is much more common (depending on the territory). It also results in less wear on the cars (lower maintenance cost, too), but can have negative effects on the rail.

    Depending on the operating scenario and the equipment used you might want to bring your trains to a stop using dynamic braking sounds. Or you might be in the throttle a bit dragging the train up to 400 feet from the signal. Just depends on the territory, equipment and individual engineer's preference.

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