Came across this IC while poking around the web and thought I'd give it a try. It's an NSI50010YT1G and comes in an SOD123-2 package, so it's something of a bear to solder. What's neat about it is that is provides a near-constant current of 10ma, and can handle an input as high as 50V. This makes it pretty useful for constant-intensity lighting. The only other component you'd need is a diode in the 1n400x series to protect it from reverse voltage. Here's one that I wired up for testing purposes:

I have some older MRC tech4 200 throttles and the DC ripple causes annoying buzzing in my HO Kato locos, and only the first 40% of the throttle is usable before the locos run too fast. I've decided I want a simple potentiometer controlled transistor throttle. Voltage source will be a large 6V pure DC battery, output needs to be a variable 0-6 volt pure DC output. That means no DC pulse or PWM systems, and I'm not interested in DCC or dead rail. Load will typically be 50-250 mA, but possible spikes up to 1 amp.