I like running my trains on DC track voltage. It gets rid of the pesky mechanical reverse unit and I can dynamically control the direction the train travels. However, I like running the action cars off of AC voltage. I have found out two things that I will share.
1) When a track trip is on a mainline: I use a dedicated transfomer that I have put in phase with the track transformers. I connect their base posts. Now I can run an AC lead from the dedicated transformer to the car trip. The action car energizes when I push the car trip button and the current returns back via the common base post rail to the dedicated transformer. AC and DC voltage can co-exist on the same carrier.
2) When a track trip is on a through siding: When the action car is set out on a through siding and the switches are thrown to the mainline, the common base post connection is broken. I have found that I can use yet another dedicated transformer that I put in phase with all other transformers as a common practice. I do NOT, however connect the base post lead to the other transformers. Instead, I solder a wire to the base post rail on the through siding and take it straight back to the base post of that dedicated transformer. When I activate the track trip with the button, the car energizes and the current flows through the base post rail to the base post on the dedicated transformer via that wire. Also, if one of my locos is on that through siding when I activate the car trip, it stays motionless when power is applied to the mainline. This is because the base posts of the mainline transformer and the dedicated transformer are not connected. When I move the switch frogs to their divergent route, the base post rail connection is re-established and I can run the loco out onto the mainline. Obviously here again, AC and DC voltage can make nice with each other on a common carrier.
To confirm that I haven't set myself up for an electrical disaster, I recall that that is how vintage American Flyer on-board whistles operate, by sending a DC pulse over a common carrier with the AC track voltage.