In here are some designs for circuit boards for my intercom circuit, with example photos. They were sent to me by someone who found my circuit and designed their own boards for them. My own boards were made using (the equivalent of) veroboard.
They did have some problems with their circuit, but I couldn't see anything obviously wrong with the design, and I haven't heard back from them since I made some fault-finding suggestions (so I presume they got them working).
I haven't tried making them, only tracing out their design and checking that it seems the same as my working ones (built many years earlier). So, if you have a crack at making these boards, carefully analyse the board against my schematics (and I can't recall if they made them for dynamic or electret mike inputs, but I think they followed my diagram for dynamic mikes, so check that, too). I'll look at them again, and update this note, later.
If you make these boards and they work, or diagnose the fault, let me know and I'll update these files.
My debugging notes were along the lines of:
Use shielded leads for the mike input that's not a multicore with other wiring (so the mike is shielded from the other wires, and the outside world).
There's a lot of gain between input and output, and oscillation can occur with close wiring. Upon reflection, I should have designed the headphone amp IC2 with the + & - inputs (pins 2 & 3) the other way around, so that the mike input is inverted with the main output, to avoid positive feedback loops (though I didn't have to do that with mine, but I used shielded wiring for my mike connection).
Initially I had instability problems with the intercom until I did the supply decoupling and filtering involving R1, C1, R2, C2, R3. That killed it rather well. You could check that C1 and C2 are okay.
On your photo of the trackside, there's a thin line snaking from the input line to the switch. I can't tell from the photo if that's a scratch or a copper trace that needs scraping away.
Having a look at the diagrams, they look right to me. Though, note the comment I put with my diagram that if IC1 is of the 5534 series, then a 22pF cap should be put across pins 5 & 8 as close to the IC as possible. It's not drawn on my diagram, just a comment below it. The omission of that cap could cause the IC to go unstable.
The intercom's audio amplifier can be tested by connecting a line level signal to either the program or intercom line inputs (a suitable source could be the headphone output from any music player).
Similarly, the output from the mike pre-amp stage could be tested by connecting the intercom line to any line-input amplifier (e.g. ordinary stereo system, amplified computer speakers, etc).