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Locomotive Momentum is a decoder feature which attempts to replicate the mass of a real train, that is, real trains just don't stop and start suddenly. Acceleration and deceleration will occur over a period of time, independent of how fast the throttle is opened or closed. Just like a car does not skid to a halt when the throttle is closed.

[edit] Train Mass

When a real train starts or stops, it does so slowly because of the of mass involved. The engine may be applying full power, but it takes awhile to get the train moving to top speed. As more mass is involved, more energy (in the form of a pulling force) required to get the train moving. To stop a large, heavy train, the forces that keep it moving must be counteracted. In this case, brakes are used to dissipate the energy contained in a moving train by changing it to heat.

With our models there is very little inertial mass as the mass of locomotives and rolling stock is relatively low. This allows our models to get up to speed fairly quickly, almost instantly.

[edit] Decoder simulated momentum

Because some of us don't want our trains to stop on a dime, manufactures have implemented "Locomotive Momentum" which helps our models to act like like real trains. Since momentum is the force we're dealing with, momentum is the term we use to identify the feature to make model trains react slowly to Acceleration and Deceleration commands. This simulates the heavy mass of a loaded train. Without simulated momentum, a model can achieve full speed almost instantly. Obviously, this isn't how real trains run.

[edit] Should I use this?

There are three primary ways most people think about this 'feature'.

1. Those that want realistic operations, but not have to worry about fine controll.
2. Those that want realistic operations, but want to control the train using 128 steps instead.
3. Those that don't know about this feature, or don't care about realistic operations.

[edit] Realistic Operations

For those wanting realistic operation to be handled by the decoder, instead of you steping the decoder slowly through 128 speed steps, then you'll want to set CV03 (acceleration momentum) and CV04 (deceleration momentum). By default, decoders are to 0 for these two CVs.

Acceleration momentum keeps the train from starting the train faster than it realistically should start, deceleration momentum keeps the train from stopping faster than it realistically should stop.

With OPS-Mode Programming, you are able to change momentum control on the fly. This allows you to simulating adding/removing cars/load/weight from your train. To disable momentum control, simply set CV03 & CV04 to 0.

[edit] Why not to use it

If you want absolute control of your trains, then this is not for you. You can still easily achieve realistic operations using 128 speed steps, as it's nearly impossible to distinguish one step from it's neighbor.

Also consider that acceleration momentum can be frustrating for some people who want absolute control, deceleration momentum can be devastating for some as their trains are not stopping soon enough to avoid an obstacle or collision. (Some throttles have an emergency stop feature just in case...)