My experience

I will relay my experience with tiny incandescent bulbs and resistors.

I computed the resistors to use like this ...

I always started with the ohm rating and then went higher still and checked the brightness of the bulb. I went for a medium brightness -- not too bright and not too dim.

I then installed the bulbs using a dab of silicone rubber cement so I could change out the bulbs if they burned out. 

In my experience, allowing the bulbs to burn continuously during times I had the layout on (8-12 hours per month), the bulbs would last 1-2 years before they would burn out and need replaced.

I finally got tired of the constant burnouts and moved to white LEDs and never looked back. I can get pre-wired white LEDs for a very reasonable cost -- and my time is better spent on other projects rather than replacing incandescent bulbs that keep burning out.

I know you don't want to hear this, but ditch the incandescent bulbs and go with LEDs. The lack of burnout hassle is worth every penny.

@David re: Bulbs and resistors

Hi David,

The accessory "port" from a power pack could mean a variety of things.
What is the make and model of your power pack?

Is the accessory "port" marked as + and -? Many accessory ports were putting out AC not DC power and as such would not have, nor be marked as + and -.

Are the lights (those little lights with the black base on a sticky pad) you are using marked with a + and -? The ones I am familiar with are small incandescent screw base bulbs that would not have a + and -.

All these bulbs (they all sound like incandescent bulbs, with a question for the crossing lights) can have different voltage and current ratings. Let's only deal with the voltage ratings first. The accessory port on the power pack usually puts out a "constant" voltage, usually AC in the old days. Unfortunately, the accessory voltage was usually approximate, and not well controlled, called "regulated" in power supply terms. But you need to find out what this is, either by looking up the specs, or by measurement with a voltmeter. correspondingly, unfortunately, your bulbs could have widely different ratings like 1.5, 3, 5, 6, 9, 12, or 14 Volts. Attaching a 3 volt grain of rice bulb to a 14V accessory terminal will blow out the bulb almost immediately. Attaching a 6 V grain of wheat bulb to a 12 V accessory terminal will likely generate a bright light for a short period of time before the bulb burns out (and gets hot in the process).

Now for the guesses:

Assuming you have an older power pack, your accessory terminal is likely putting out 12 V AC not DC. A newer one might put out DC, maybe even 12VDC. (This assumes HO scale models and power pack).

Depending on the size of your grain of rice bulb, it could be either 1.5V or 12 V. They each can draw as much as 300-500 milliamps (0.3-0.5 Amps) but are more likely to draw 0.15-0.3 Amps)

Your crossing flasher has 3 wires using 2 lights because one wire for each light is connected together and the one wire is brought out as a "common" connection. They assumed you would want to flash them alternately. If you can't tell what kind of lights are in the crossing signal, you need to be wary, as they could be LEDs (Light emitting diodes) which will want only DC at a low voltage each with a dropping resistor like 2200 Ohms 1/4 Watt.

An investment in a cheap voltmeter would help tremendously.

An alternative would be a low cost investment into a range of 1/4 or 1/2 Watt film or carbon composition resistors like 470,560,680,1000,2200,4700,5600,6800,10000,22000 Ohms to be used (from high values to low) when you don't know the output from the accessory port and don't know the characteristics of your bulbs.

You start by putting the highest value resistor is series with one light, hook it up to your power source, and note the brightness. As you reduce the resistor value the brightness should increase. When it looks like you see no appreciable difference than the last value, you have likely gone too low so up it one or two values.

A better way is to figure out what the ratings are for all of these and match the right bulbs. You can still use the same resistors to lower the brightness to a pleasing modeling level, and by doing so, you will radically extend the life of the bulb. Using bright bulbs means they will get hot. Using hot bulbs with small plastic housings and shades, will likely deform or melt the shades.

'Hope this helps. Ask more questions if needed.
Have fun! 
Best regards,
Geoff Bunza

Lights, multimeter, action!

Joe and Geoff,

I have a multimeter and now I really have a better idea of how to use it. Thanks for the replies. The little lights with the black base are from Micro-Mark (pkg of 25) and have a red and white wire that I am guessing is positive and negative. They have worked well on several buildings from my old layout.

The small grain of rice ones have two black wires and were parts in kits for stairway lights etc. These I will have to test and use a resistor I am guessing. The power pack for lights is a "Troller" with three electrical ports, one of which is labelled AC Accessories. It was used on my very first HO layout and had momentum control as part of the slide switches on the controller face. The accessory port is where I had building lights hooked up (some of those small black base sticky ones) that were medium brightness and still light up today when tested with a 9 volt battery.

Not so sure about the crossing flashers - as I remember, I had hooked them up on the old layout and they both stayed lit. Do not have a "flasher" unit that they hook up to and the rail for activation of an approaching train. Thinking about it, but already have a gazillion wires under the layout now with power districts and auto reversers etc. Things were so much easier in the military when all I had to do was blow things up. . . . . Smart thing was to label each wire and terminal strip with my wife's label maker.

Okay, will trudge on from here with your advice after rereading it several times to further my electrical education. Again, thanks and Joe, this site is the best thing since sliced bread for me. Really helps those of us with 5 thumbs. . . .LOL!