I want to build a replacement for this "whistle relay" in the attached picture. Between 0-24vac is usually present. When approximately 20vdc is applied to the track, the relay closes and the ac voltage is allowed to flow to the couplers. (As a bit of background, Lionel produced this in the late 1930s. In the transformer there is a rectifier that produces the dc. The dc relay usually blows an air whistle, but it sometimes is used to cycle the e-unit to change direction of the motor, or to operate the couplers.) This mechanical relay is still available, but I would enjoy building my own. Here is a picture of the part that does this in modern trains (basically what I'd like to build). Because the ac and the dc are both on the same circuit, I believe I cannot use a single relay? I did a lot of searching and looked through the online textbooks, but I'm still lost. Thanks!