You can get super-bright, white LEDs in sizes up to 10mm now from a couple of sources:
http://www.moreleds.com
http://www.superbrightleds.com
By all means, go with the LEDs. You can get the larger T-1-3/4 in gold, white and warm white to represent the "type" of lighting you want to represent. The high output LEDs are extremely bright .... they use them for flash-lights now !!! The best part is the fact they require minimal current to operate, generate no heat and will last forever !!!
If you find the focused lense on the LED produces too much of a pin-point beam, just lightly sand the end of the case .... that will disperse the light more evenly.
Mark.
¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ
Very good advice !!!
I strongly recommend against trying to use the halogen bulb. It could create a potentially dangerous situation. Heat is the biggest factor - they get very HOT and could cause injury if someone touched it, or if it fell over and came in contact with anything remotely flammable. I've read cases where household halogen lights have actually ignited curtains that were too close to the lamp !!!
Best to err on the safe side.
Halogen bulbs have a high current draw. They also come in various voltage ratings. If the 5 Watt bulb won't light up on the power supply you have, it probably doesn't deliver enough amperage to light the bulb or the bulb requires a higher voltage.
Is this searchlight mounted on a flatcar? A DC power source is not as good as AC for any type of filament bulb such as the Halogen you're trying to use. The track buss should not be used for the bulb because that is a variable voltage and the bulb will draw too much current away from the locomotive(s). The heavy current draw through the wheels and axles can cause them to overheat and fail. The AC Accessory terminals on the power pack would be better, but are not going to be usable for the searchlight if it is on a piece of rolling stock.
Halogen bulbs, their sockets, and their wiring, also get very, very hot and will melt any plastic within an inch or so of their location, so a different type of bulb would be advisable. You must also be extremely careful when handling a Halogen bulb and never touch it with bare skin. The oils from your fingertips can cause the bulb to explode, so they should always be handled with gloves. The likelihood of a Halogen bulb exploding close to your face and blinding you is too much to risk.
Read these Safety and Handling Precautions about Halogen bulbs from Wikipedia:
http://en.wikipedia.org/wiki/Halogen_lamp