Using a Darlington (ULN2803,etc) driver is really easy way to drive things that are close by, example engines, steppers LED's matrices, etc... yea.. further away too, but..
Now as a fan of model railway, I know that rewiring or putting in more wires can be a pain in the back. Usually you already have the wires there- you just need to switch them. Using a solid state relay can be a simple and quick solution and if you search around you can get them in packs for reasonable price.
All you need to do is connect the LED light you want to the existing wire on an Arduino output and by the LED connect it to the solid state relay Sensing Pins. Using your existing power source of whatever Volts you put the live on the Switch Pin Supply. Then connect the existing LED power supply to the Output Supply.
Some people may thing.. umm that is the same what we jsut answered using the Darlington.. well- at least you got another option in terms of wiring and expandability.
Example of SolidStateRelay