The more I work with DCC, the more I'm rethinking a lot of my operational approach. The conventional wisdom has been that DCC is best for allowing operation of multiple locos without the need to isolate block power. However, I almost never operate other than by myself, but I find so many advantages to DCC besides operating multiple locos that it's raising a lot of questions.

One is that, although a 4-function decoder will allow you to run a loco with headlights on or off, there are other options, both in the age of oscillating warning lights and in the age of ditch lights. A 4-function decoder won't allow separate control of ditch lights, or allow them to flash (or not as the operating rules provide) at grade crossings. This is a problem both with decoders and with light bars. For instance, Kato light bars allow only one option for headlight and ditch lights, all on or all off. There's no point in installing a 6-function decoder in the Kato locos I'm familiar with (I don't know if the new AMD-103 is different), since the light bars don't feed off separate LEDs.

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The first high-end Walthers Proto loco I've looked at is the GE U28B, and I'm delighted with the options you can get using the 9-pin plug and an NCE D15SR 6-function decoder. Ordinary upper headlight and number boards lit:

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Lower headlight only:

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Lighted number boards only:

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All lights lit:

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What I noted back in the days of oscillating warning lights, both from memory and from DVDs, was that use of these lights wasn't consistent. The engineer could run with just the headlight, with both lights lit but oscillating light not oscillating, or headlight and oscillating light. By the same token, ditch lights are used in normal operation on the main line, although if the train is stopped, only the headlight is lit. Headlight is lit toward the rear on DPUs. In yard, a moving loco may or may not have headlights lit, no ditch lights.

I need to keep studying all this, as well as understanding more clearly what the commercial models and decoders are capable of. I want to avoid Kato style situations -- or for that matter, Atlas Trainman Genset situations where there are ditch lights, but they're all wired together, and there's no good way to rewire without a lot of work.

I'd like to hear anyone else's experience along this line.

You just open a can of worms.
There are several very different aspects to consider
a. How many physical outputs provide the decoder.
Most decoders provide four outputs (white, yellow, green and brown). That sounds good but you are reaching the limits very fast when you want to have front and real lights and alternating ditch lights. All the different lights SP used to have will also exceed this wide found capabilities. You may have to look for not so common decoders to drive more than four lights. Some decoders may provide four wires and "some" tiny solder pads on the small decoder PCB to solder your own additional wires (or destroy the decoder if you do a solder mistake)
b. What logical function can be assigned to why output
Most decoders use directional control for the white and yellow lines. Green and brown are of universal use. But you have to read a lot of details of the decoders to understand if each line can a strobe, a blinking light etc. There are a lot of none standard implementations available on the market
c. What physical lights sources are available on the model
Most models have only one front and one rear light. They may "drive" the front/rear light and the number board. Ditch lights may be available but wired in parallel (Atlas Genset). Most not very modern models lack ditch lights at all. You will find a huge number of instructions how to modify and/or add lights in the internet.Some are simple but most are fairly complex. Functioning ditch lights require in most cases SMD LED soldering and some very fine shell modifications.
d. Do you want to have the best decoder suitable for each engine?
I hate it to find a new set of CVs in each engine. I want to use one line of decoders of one brand in all my engines to do reprogramming in my familiar CV environment. You may not find a decoder brand that fulfills all your wishes in all of your engines.
e. Lights are great but there is something more
The decoder has to drive the motor. You will notice that motor control is implemented very different by the different brands. Especially best motor control of low cost motors limits your choice of brands dramatically. Virtually every decoder will run fine with a Trainman GP38 drive but what do you do if you "fall in love" with an old brass or low coast model with a lousy motor.

Just some thoughts based on my DCC experience.

So far, with a limited experience of installs, I'm preferring NCE and TCS decoders. I'm also seeing that the NMRA 8-pin socket is limiting in the functions it can provide, so I'm naturally going to lean toward locos with 9-pin sockets -- although if it's a prototype with just a single regular headlight, it doesn't really matter, and if there's no other choice, there's no other choice. What this will probably impact will be which locos I put in the lead in consists, where I begin to have a variety of possibilities.

I've opened a new MTH sound-equipped SD70ACe. The lights are interesting, but unlike the 6-function DCC decoder I discussed above: the headlight comes on dim at startup, can be turned on and off. Number board lights can be cycled on and off with F5 DCC. However, the headlight comes on bright and the ditch lights come on only when the loco starts moving. I don't believe this is how the prototype works -- I think the engineer turns the ditch lights on as a separate step. The positive is that the ditch lights do operate independently, and you don't need a separate F key to work them. The negative is that the function is automatic. Still, it's one less thing you have to think about for most prototype operation, and the loco does have a headlight lit on a siding or in a yard when stopped. I don't think, though, that you can have a headlight lit in reverse if this SD70ACe is running as a DPU -- I wish you could get this option.

Clearly there's no standardization among these features! However, I think the MTH loco is a better value than the Kato SD9043MAC due to the additional lighting control.

With the MTH SD70ACe running, I was emboldened to pull out my MTH DCC-ready GP35. It turns out this is a big problem. The coupler mount is a rube Goldberg deal, and the body basically can't be removed from the chassis without breaking the Atlas style tabs (there are several threads on various forums that confirm this). Then when you get to the PC board, you basically have to obtain a 6-function decoder with an 8-pin plug and extra wires hanging loose that can be soldered to contacts on the PC board not covered by the 8-pin socket.

Doable, but I will need to order one of those next month.

Again, there is no standard and no easy way to handle some of these things!

While this isn't related to special lighting effects, another thing I've discovered is that the current run Atlas RS-3, while it now has an 8-pin socket, has no room to mount a decoder other than the type that is directly attached to an 8-pin plug like the NCE D14SR.

Again, there is no standard and there aren't really good guides for how to handle these issues on later-generation models.

Well, I installed a Digitrax DH163P decoder in my MTH GP35. (It's worth pointing out that I got this from a guy dealing decoders and other electronics at a swap meet. Neither of my two closest LHSs really wants to carry DCC -- I assume they figure the markup on individual decoders isn't enough to make it worth their while. But neither do they stock the bigger-ticket items like base stations or boosters. I think Fred Hill in particular is a jerk for thinking this way. Hey, why not have goys knowledgeable enough to do installs and charge for that? Oh, I see, you'd have to pay them more.)

Anyhow, while I can control the loco and turn the headlight and class lights on and off, between the way MTH has the wires running to the PC board for the various lights and the CVs that Digitrax uses to control the function keys, I have yet to make the thing have a constant headlight in the lower position and a mars light in the upper -- MTH is a big mystery. However, SP often enough ran its locos with the mars light as a steady-on that I can live with how it is now until I figure it out.

I also have both a Walthers CB&Q and a WP U28/30B. These run extremely well and take a 6-function 9-pin decoder with no fiddling -- AND the headlight, number board lights, and mars light (both prototypes had them) are separately wired and controllable with CVs. Not only that, but I could re-map the functions so that the CB&Q had its mars light in the upper position and headlight in the lower. THERE is a good reason for DCC, though some manufacturers don't provide for the function by wiring lights separately.

I'm learning a little bit at a time -- and as far as I know, there's no good central place to learn this stuff. Extension classes in hard knocks, I guess.

Lesson for today: Athearn RTR has all the bulbs wired together, so that Mars and headlight aren't separately controllable. There are posts on various forums about how to customize this, but that's not something I want to spend a lot of time on. So what you see here on an RTR SD45T-2 is what you get:

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I assume this applies to D&RGW SD40T-2s and SD50s as well. This is acceptable, because both SP and D&RGW sometimes ran with Mars lights lit but stationary.

Genesis locos, on the other hand, do have separately controllable lights, although since there's no standard on this stuff, F1 controls the Mars light on an Athearn Genesis loco, F2 on a Walthers.

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I suppose this is something I could remap.

I'm discovering that every DCC install is a little different. My wife gave me a gift card for my birthday that let me get some decoders to install on a couple of my P2K pre-Walthers GP9s. These had the cracked axle gears, so I replaced those and then went ahead with the DCC conversion. I blew the 1.5 v bulbs that P2K had for DC constant lighting -- took me a moment to remember that old P2K had 1.5 v bulbs that have to be replaced for DCC. So I got some Miniatronics 14 v grain of wheat bulbs, and the replacement went well. Here is a chassis with new axle gears, a Digitrax DZ126PS decoder, and new 14 v bulbs:

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Here's one of the P2Ks with shell installed:

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With the axle gear issue fixed, a decoder, and DCC lights, this looks like it will be a great loco. I now also have a routine worked out for converting my stash of old P2K locos with the cracked axle gears.