On the prototype, the engineer notches up the throttle, the prime mover starts ramping up in speed, the generator is able to generate more power, this power transmits to the traction motors and the loco starts pulling. Once the tractive effort is applied to the rails, the force is applied to the 5,000-10,000 tons of train, and slowly accelerates. All this takes time and delays the startup of the train from when the throttle is originally moved. Energy has to be put into the train to start it moving, slowly at first, and then more quickly. All the while, the prime mover is screaming along. When stopping, the opposite happens. Shut off the power, and the momentum of the train will continue to carry it forward. Until friction, either by the brakes or by the train itself, overcomes the momentum, the train will continue to roll.
Typically most model RR's like to just spin the knob and the train respond immediately. And without momentum, you cannot simulate the sound of the momentum. The reason the engine does not 'load up' before moving is because there is no way the prime mover sound 'knows' that you intend to move before you spin the knob and expect the loco to move. Or said another way, you need momentum to simulate a heavy locomotive.
One solution is, as you mentioned, do manual notching. Basically this allows you to adjust the prime mover sound manually while running the loco. Not very fun, and not very realistic. A lot of function key pressing is required to play with the sound, not something a real locomotive engineer would ever do. Besides, if the operator gets busy, they will tend to not do the manual notching and all the realism goes away.
The other option is to set up momentum so that there is enough time for the prime mover spools up before the loco start to move. Simulating this takes tweaking a number of CV's in the TSU. There are CV’s on a DCC decoder to simulate this. This applies to both startup/speed up of the train (acceleration), and slowdown/stopping of the train (deceleration). With high CV 3 and 4 settings, the prime mover has to spool up before the train really gets going, and similarly, the prime mover will spool down to an idle before the train stops. The Tsunami can reproduce these effects very nicely.
When programming these values, you need to ask yourself whether you want the train to stop on its own when the throttle is shut off, or do you want to require another function – BRAKING – to be required to slow the train. If you want the train to stop on its own, just select a CV4 value that slows the train in the distance you like. If you want braking, you will need to set CV4 to a very high number, then set the braking function (CV61 in the TSU) to slow the train at the desired braking rate.
As was mentioned, there are 2 key variables that one needs to play with to set momentum:
CV3 - Acceleration
CV4 - Deceleration or baseline braking rate
Additionally, if you want to use the braking feature, you will have to activate the brake function on the TSU - CV61. This CV basically "accelerates" the deceleration rate, or in other words, reduced the deceleration factor as a ratio of the deceleration value in CV4 to what you program in CV61.
So putting it all together, to make this all work, program a reasonably high number in CV 3 to simulate the weight of starting a train, put an incredibly high number in CV 4 to simulate the train coasting, and pick a value in CV 61 that will let you stop your train in a realistic distance. Some numbers I use:
CV3 - 65
CV4 - 175
CV61 - 250.
*** UPDATED!!! *** READ ON!
I often have newby operators on the GNW. These settings work OK IF people are used to them. We have had a few run-away trains and the brakes were simply not effective enough to stop the train in a reasonable time. So operators were simply hitting the “E-Stop” button to stop trains – not exactly realistic. SO… I went through ALL my road locos and now use these settings:
CV3 - 65
CV4 - 60
CV61 - 160.
Much more controllable, and still offers enough lag to let the prime mover spool down while coasting to a stop. Brakes provide good control in switching and around yards.
On a TSU, speed tables will have to be activated make CV61 work. I like to use a value of 50 which gives me 28/128 speed steps, normal direction, DC not enabled and speed curves active. Note that the braking function is programmed into F11 on the TSU, somewhat awkward to get to on most handhelds. I remapped that over to F7 to make it easier to get to when operating, and it matches the default programmed location on QSI locos. (CV41=128).
Here's how it looks in a video when you put it all together. Note that I did not use manual notching or hold the brake to get the throttle to sound realistic. The locos are programmed to be run like a regular loco - and respond as if they each weighed in at 250,000 lb...
Sorry for the low quality of the vid. I hope to redo it sometime. Note how the locos coast into the yard. I am turning F7 on and off to modulate the braking and stopping of the train; no throttle at all - it's just at zero when I get near the yard.
One other key for this to work right is that the starting voltage and slow running characteristics have to be tuned. There are a number of procedures for this, and before you mess with setting up momentum, you will have to get that tweak done. Check out my other pages on the website for setting up a decoder for slow speed.
Keep in mind many operators do not like a lot of momentum in their locos as they want it to stop when the kill the throttle. That is a tradoff on these settings. With the momentum set prototypically, you will not be able to stop on a dime, just like the real thing! I like the added challenge of it. It adds 'train handling' to 'railroad operations' on the layout. I like that. Others do not. Initially I had both yard switchers and road locos set up for this and my crews went on strike. Now I have just my road locos set up with this and the yard engines have minimal momentum - CV 3 and 4 = 15 or 20. It works OK for that. It also slows people down when running a train - no one wants to plow through another train or miss their stop. Again, I like this and I feel it results in more prototypical operations. Your mileage may vary.
One other point. On the prototype, notch 1 actually does not spool up the prime mover, only connect the generator to the traction motors. In this mode, only 'idle' power is transmitted to the traction motors. This is usually enough to move the loco around the engine yard or stretch the slack. TSU's used to have their decoder set up so it would allow going into speed step 1 without spooling up the prime mover. From emails dialogs with them, I understand they got a LOT of complaints from people who ~felt~ it did not sound realistic to have the loco move without the PM notching up. All the newer TSUs notch up as soon as you come off speed step 0. Too bad. I still have a couple of those older decoders and I tell you, I love hostling those engines around the yard where they don't change pitch or sound when just creeeeping along. Lot's of good memories watching that action on the prototype growing up....
Hope all this made sense.