Standard 8mm 12V strip led lighting offers several advantages for cab lighting. For one thing it is usually much faster and easier to use than wiring up individual leds or GOW bulbs. Also it is super easy to fasten to a diesel or steam cab roof being self-adhesive. The leds are nearly flush against the cab roof totally out of normal eye view.
Here are some variations I have used. Note they are all based on the standard 300 led per 5m reel using 3 series leds every 2” of length. Warm white works well for most situations but you can also use white if a more bluish hue is favored.
Type 1 is a regular 3 led section designed for 12VDC, which is quite bright. A more realistic brightness for our use is about 9 to 9.5VDC IMO, like used in passenger car lighting. You can also power this strip directly from the track at up to 18VAC command power, but you must add another load resistor in the wiring in the 750R to 1K range.
Type 2 is for use with low voltage TMCC lighting. You will note the jumper across the middle led and the series resistor. This effectively takes them out of the circuit so that you end up with 2 series leds with no load resistor, spaced about 1.33” apart. So the load resistor must be included in your wiring somewhere. These fit nicely in F3/F7 cabs and most newer diesels placed right across the cab roof, such that the leds are situated just above each of the engine crew. These have the advantage that they will produce light down to about 5.5VDC using a load resistor in the 100-150 ohm range.
Type 3 is especially for narrow cab units or steamers where you still want 2 leds. Note in the upper left corner I have scraped the white top coat away to bare the 12V+ copper runner and the adjacent led foil. A blob of solder between the two and you have a complete 2 led series circuit. Cut off and discard the left end and you wind up with two leds spaced about .667” apart. The whole thing is just over an inch long. Again, a load resistor must be included in your wiring; these make light down to about 5.5VDC with a 100-150 resistor.
Have fun! 
Rod
