18 VAC Motor Control

I would greatly appreciate someone to help me out with this design. What I am doing is controlling a Lionel train with a simple triac/diac (actually a Quadrac) type control for a fan motor at 120 VAC. I can wire the control on the primary side of my 120 VAC to 18 VAC transformer and it works fine to control the train. My problem is I don't want it on the primary side I want it on the secondary 18 VAC side. I can run multiple trains from this transformer individual controllers for each train. The breakover voltage of the diac is too high for this lower voltage. I must start from scratch with the control and don't know if I need zero crossing detection or not or if I will need a opto-isolator or not. In my small mind this shouldn't take much of a circuit but my background isn't electronics (have alittle knowledge), it's in control wiring. Could someone post a schematic or two on how to accomplish this control. I have attached a schematic of the fan controller mentioned above. This is the type circuit I am looking for, the internal diac breakover voltage is min. 33 volts. Maybe this controller can be modified to work on the secondary low voltage side. Thank you.

BTW: Train is in fact AC powered at 5 amps max.

You might try a standard light dimmer that uses just an SCR with no Diac. They should work down to a lower voltage.

Most light dimmers are mainly for resistive loads not inductive. Would have to add snubber, might as well build something. I just lack the knowledge to modify what I already have.

Okay, tried standard light dimmer, does not work.