Rule 17 is a prototype railroad rule specifying conditions for dimming lights on locomotives. Many DCC decoders support this dimming to a greater or lesser degree.
What is Rule 17?
Rule 17 is actually a collection of rules that govern which lights are to be illuminated on a locomotive at given times and to when they should be be dimmed. These rules may vary from railroad to railroad but generally follow a similar pattern. The general purpose of the rule is to make sure that the locomotive is visible, without creating undue glare for others in the area.
- Except when an engine is clear of the main and stopped, both the front (and rear if so equipped) headlights should be on.
- The light in the direction of travel should be a full brightness except:
- 1. At stations and yards where switching is being done.
- 2. When the engine is stopped close behind another train.
- 3. In non-signalled (dark) territory, when the engine is stopped on the main track waiting for an approaching train.
- 4. When approaching and passing the head end and rear end of a train on the adjacent track.
- 5. At other times to permit passing of hand signals or when the safety of employees requires.
- The opposite light should be dimmed.
Rule 17 is commonly used as the model railroading term, and was the traditional prototype number for this rule. In modern rule books it is usually covered as Rule 5.9.1 to 5.9.4.
Rule 17 and DCC
Many decoders can support Rule 17. Configuration variables must be set, which will vary between decoders. Lights must be set for independent operation -- F0 controls front light, and F1 controls the rear. Since Rule 17 specifies behaviour of both lights, it is clearly not compatible with directional lighting. If Rule 17 is enabled, the light opposite the direction of travel will be dimmed. An additional function (usually F4) will dim the light in the direction of travel.
There will usually be another CV to set the brightness of the dimmed light. This will usually require tuning, particularly in non-"plug and play" decoders as the duty cycle necessary to dim a LED will be different from that required by an incandescent bulb.
Some decoders will automatically dim when stopped, which is convenient if not exactly prototypical. Others require the used of F4.
Also see FX Lighting.