If you always have the same number of block signal bulbs on at any given time (albeit switching colors) you can measure the current and then use a resistor to drop the required voltage. So the voltage drop changes with load.
The advantage of the diode method described above is it drops a relatively constant voltage irrespective of load.
A variant of the diode method is to use Zener diodes which might allow you to use just two components. Zener diodes comes in values like 4.7V, 5.1V, 5.6V, 6.2V, etc. By hooking up two in series in "opposite directions" as shown, the pair drops about the rated voltage plus an additional diode drop (of about 0.6V). Depending on the number of bulbs, you might use, say 1 Watt zeners which you can get 10 for $1 with free shipping from Asia on eBay (search "5.1V zener diode").
In all above cases, you need to insure the components can handle the power (heat) thrown away in lowering the voltage. As mentioned that depends on how many bulbs you are driving and knowing about how much current each one draws. Obviously you can just overwhelm the issue by using large multi-watt parts but this can get unwieldy.
Or, if your lamps can operate on 12V DC, consider converting your AC transformer output to 12V DC using a LM2596 (or similar) voltage regulator which is much more efficient (less wasted heat). When all's said and done, this might actually be cheaper and more compact than above methods but again depends on ability to use DC to your bulbs. There have been many threads on OGR about these voltage modules.
