If you can stand a bit more complexity (and have space to conceal one AA or AAA battery), here’s a circuit which will provide constant illumination even when the train is STOPPED. The battery provides power to the lamps at all times, and recharges whenever the train is moving forward (DC cab). No ballast lamp is needed here, but the resistors can get a bit warm…so make proper allowances. The battery should provide at least 20 hours of illumination between charges. If it should run down, I suggest setting the depleted car on a siding all by itself (or with others needed recharging…but no locomotives) overnight with full throttle voltage applied; the bulbs will glow, and the battery will recharge. Take care in connecting the diode (D) to the rail; it should connect to the rail which has a positive voltage on it when the locomotive moves forward. Otherwise, the battery will charge only when the loco is moving in reverse.
Note that DCC layouts gain no particular advantage from this circuit, as the DCC carrier voltage is on the track at all times; hence, the first circuit on this page is probably the best choice for such layouts.
Want a lighting circuit that will “charge” itself no matter which way the train is moving (as long as it IS moving)? The two circuits below will do it. The “Deluxe” circuit uses a Memory Back-Up Capacitor, a super-high capacitance (up to 1 Farad) little jewel commonly used to maintain dynamic memory when power is off. They’re quite compact and reasonably priced (under $3 for 0.47F, and under $5 for 1.0F). Depending on the voltage on your track when operating, this circuit could keep a car illuminated for up to one minute when the train is stopped. Use the 18-001 15mA bulb unless you must have the slightly higher brightness of the 18-201 40mA bulb.
I’ll readily admit that the above circuit has grown a bit complex. If you’re the adventuresome sort and prefer the memory capacitor to batteries, you might want to try the variation below. Here, we’ve replaced the second (low-voltage) regulator and its associated components with a “ballast bulb,” which is simply a 12 volt, 40mA Miniatronics bulb that serves to consume the voltage not needed by the 1.5 volt bulbs.
The “Super-Deluxe” circuit above uses batteries to keep your cars lit for hours. Three Nickel-Metal Hydride (NiMH) cells work splendidly…if you have the room for AA or AAA cells; if not, consider using one of the many battery packs sold as replacements for cordless (NOT cellular) phones — they’re usually 3.6 volts, and more compact that the AA or AAAs (AAs could last up to 15 hours, AAAs up to 8 hours, and a phone battery pack 2-3 hours). To change the brightness of your lamps, simply change the value of the 39 ohm resistor shown in these circuits; the chart below suggests resistor values for more or less brightness. Remember that more brightness means less bulb life, and vice-versa; choose wisely.