The electronics behind flickering lights is very easy to assemble. I got the inspiration from the May, 1992 Model Railroader magazine article "How To Simulate Flickering Fires" on page 72. The key component is a transistor radio with a headphone plug. You can find these in garage sales, bargain bins, or on electronics surplus company web sites, which is where I believe I bought the radio shown in the photo here.
A cheap set of headphones is the other part needed. Cut off the headphones ear pieces. I made the wire as long as possible so that I could locate the radio somewhere under the layout without having it interfere (space-wise) with all the things that normally go under a layout. The final part is an amber LED. The amount of current coming out of the headphone jack of the radio is small, so no current-limiting resistor is needed. However, you may wish to experiment with this depending on the LED you use, and the brightness you desire. I soldered the LED to the wire (polarity doesn't matter) and inserted the plug in the headphone jack of the radio. After installing the batteries in the radio, I adjusted the tuning to a frequency of a known local radio station (it really doesn't matter which one), and adjusted the volume to have the LED produce a flicker without going off or being full-on. If you have an accessories power bus running under your layout, you could experiment with powering the radio via that bus, rather than using batteries.
As mentioned above, the circuit is really simple. You need a radio, matching headphone plug and wire, and an amber/yellow LED. It is possible to attach several LED's to the wires. This would allow for flickering lights in various locations, such as in a machine shop, in a 55-gallon drum, or a burning building, all running from a single radio.