Switch machine power supply

Here is the schematic for a "snapper switch machine" taken from page 36 of Railroad Model Craftsman, January 1973. I have built this and it works well.

            ---I1----
            |       |
            |  R1   |
          ----\/\/\-------------------------------------
          |          /                                 |
       D4 _         /                                  |
          ^        /gate                               |
          | anode / cathode                            |
     o---------->|-------------------------------->|------------o
              SCR1       |         |              D1
                         |         |
                     +   |         |
 12-17 VAC              ___        _
    IN             C1    _         ^  D2                       OUT
                        -|-        |
                     -   |         |
                         |         |
     o----------------------------------------------------------o

Parts List:

   - I1:	16-20 volt bulb
   - R1:	560 ohm 1 watt resistor
   - C1:	5000 to 10000 uf 35V (or more) capacitor
   - D1:	12A 50V PIV diode
   - D2,4:	1A 50V PIV diode
   - SCR1:	6A 200V silicon controlled rectifier