PCB to combine MTH terminal block with 6 relays to control sidings/track blocks

I have done something very similar to what you have done, only slightly different in the method and wiring. It's really pretty close to what you have though. Yours looks a bit more elegant than mine and is also a bit more compact. I don't have any expansion terminals for more spurs either, more relays would have to be added on my setup. I will try to explain it, apologies in advance it it doesn't entirely make sense.

I had a problem with some Toggles with LEDs. When the switch was on everything was fine, but with the switch off the common would backfeed to the hoot terminal of the track that was off. This was all well and good until you forgot to flip a switch and ran a train on to the dead siding causing a short and blowing the breaker. After considerable head scratching I finally figured it out.

I then added relays as you have, but I used individual 12 vdc ice cube relays on din rail mounted next to my MTH terminal block. I already had the relays and din rail. I didn't have far to re-route the wires so I used a 12dc wall wart to power the switches and relays and just ran the track power hot though the Com & NO of the relay. Everything is all working properly now and has been for a few years now. This happened a while back.

Anyway I didn't make a wiring diagram (probably should have), but my setup is very similar to yours with the exception of no main block power being used, that is still all like it was before the glitch. The only thing I have on the relays is the yard spurs where trains are parked when not out on the mains. The spurs (when on) share power with one main being fed from a PH-180 via my TIU. I really never had signal problems, even when the spurs were just powered by the switches. The spurs and switches are totally isolated now so the signal should be about as good as it can be.

I have considered replacing the ice cube relays with one of those multi relay boards for Arduinos from ebay, but it's all working and I just haven't done it. It would make things a bit more elegant and closer to yours as well, but it's under the layout so there it sits.

RTR12,

Previously I was using some cheap relays from ebay, but I was not happy with the quality of the screw terminals. I sometimes had a difficult time getting a tight connection without damaging the screws. 

Thanks! I think you have just given me another reason to leave my relays alone.   The relays I had/have are fairly good quality commercial relays. Probably still made in China, but their sockets and screw terminals are quite nice and hold the relay and wires very well and have a couple of other nice features. Screw terminals would easily take #14 wire (and probably #12) with no problem. I think they are rated at 12 amps.

Only drawback is that they were probably about $8-$9 each total, for relay and socket (not including DIN rail), IIRC. Before I retired, my employer used these a lot for commercial work so I knew they held up well and lasted a long time. I suppose another drawback is also size, they are a bit larger than most of the stuff we use for our trains.