Back when I operated my analog Marklin gear, I managed to mostly stay away from sectioning the track by using the catenary to feed one loco while operating the other off of the track. It was also quite cool because if I was only running the one electric loco, I could set the track voltage to a suitable level to always keep the lights in the passenger cars on regardless of the loco power (since the cars got their lights power from the rails). Of course, it looked a bit weird when the lights in the passenger cars dimmed because I was modulating the power to a second loco that was running on the rails, but that's a different matter. I think I can safely say I got my start as an EE, setting up the circuitry for that train layout.
Now, fast-forward to the present. The old gear is getting dusted off and missing pieces are being replaced and digital equipment is pervasive. It's everywhere, even in the "grab boxes" of special deals. But if I want to probe into the world of digital train operation, I still have my old analog stock to consider. I know that there are conversion kits to make them all digital but that's not a path I'm ready to follow. At least not yet. So I'm wondering if I can get analog and digital to safely co-exist on the same track. Obviously, I can resort to rail sectioning but that's sub-optimal. I was thinking along my old trains of thought and figured maybe I could set up a digital feed on the rails and run analog on the catenary. That would have the side benefit of the old passenger cars' lights getting fed all the time.
I ran this idea by a guy in one store but he wasn't too happy about it. He'd much rather convert my old loco to digital (which, of course, is where the money was in it for him). He said he believed the analog train would interfere with the digital gear, which is possible, I suppose, but not something I'd really accept his word for just like that.
The way I see it, there would be two electric circuits sharing a common earth (the rails). One circuit would be the digital mix of constant-voltage AC with overlaid signalling in the center rail and the other would be the analog varying voltage in the catenary. Now, it's possible that the resistive losses in the rails may cause some (moderate) voltage variations due to the extra current from the analog loco, but that can be reduced by proper wiring. I don't think I have to worry about ground loops (this is hardly HiFi audio). The only major worry would be transients from the analog loco (like if there's a short break in catenary contact so there's a spark; happens now and then) doing nasty stuff to the digital equipment, but I kind of think they have to be prepared for that anyways, so isn't that a lot of what those special rails for connecting the cabling are supposed to do?
A lot of rambling, I know. I just wanted to run though my arguments and hear if there's anything I've overlooked. Thoughts or experiences, anybody?
/Tirsten